Don't start caving. A 10-year-old does not need to sit in the front seat with Daddy while his wife sits in back. A 10-year-old does not need unlimited dessert. When you let the child run the house to the point that you are being disrespected, you really have made a mistake.

You don't mention your husband's role in all this. When Princess doesn't get to kick you out of the front seat, is it YOU who has to defend your place? Is it YOU who has to tell her she can't have a 4th ice cream cone? If so, that's the problem. If it's your DH who is parenting like he should and you just happen to be there, who cares what his ex says or thinks? If he applies appropriate consequences for sulking or glaring or backtalk that turns to disrespect, let him handle it.

It's when the DH is lacking that SMs usually have issues with SDs. 

Difficulties in step life often ramp up around big life events like engagements, weddings, births etc. Countless SMs that had no problems while they were "just the gf" find a switch flips when they marry, and it's often the BM creating the strife. Even if the split was far in the past or BM has had multiple new partners, some women can't stand to see their ex move on.

Change can be hard for skids, especially when BM is dripping poison into their ears. Sometimes they feel they must complete to be number one with their parents. It's essential these behaviors be dealt with consistently by the bio parent, and that they be on the same page with their partners.

I suggest you get the book Divorce Poison & read it TOGETHER. And consider family counseling (dad, SD, SM). That's how you combat dilited parenting and BM's venom.

These stories always sadden me, because they're really about kids who are growing up feeling insecure and lacking stability. Kids deserve to have the reassurance of a firm family hierarchy, knowing where they fit in (um, BELOW the adults), and to have parents that love them enough to correct them.

Leave all discipline to your DH.   I'm speaking as a former stepchild and then later, stepmother..

You can't "get in trouble" with BM if you don't give a sh*t about what that harpy thinks.

If she's texting you, block her. She can text your husband and he can relay it to you. You do not need to interact with her and you can lay down the law with your spouse. It needs to be 100% his problem.

Every one has there place. Husband and wife sit in front of a car together.  Kids sit in back.  If SD wants to play the control game.  She control the home .. going to BM for help in her endeavor.  
'You set the rules...you are in control...  SD has a place as the child.  You are the adult... 

'BM does not control your home.  Or your child raising system...  You married a man with a child.  You knew certain things have to happen because of this..  but no one wants a outsider. BM to have any say in my home.  P once you establish the house rules things fall into place. 

If you haven't already done so, please block BM's number and block her on all social media. She doesn't need to have access to you. Don't let her near you in any way.

I wouldn't even let your partner mention what she says about you... Why? Doesn't matter. Who cares what she thinks. 

SD is not going to stop.  She knows she is causing problems with your and u DH, relationship with her BM.   BM , has nothing else going on in her life so she jumps in to hurt you.

 You must disengage from BM.  You can not let BM control the "life's: of all.  DH should parent his child .. putting her and BM in there places.  Having your back,  it's like these people have nothing else going on in there life's except causing trouble.  Your family needs a actual flow chart to show who is responsible for what.   There are two different homes with two different set of rules.  

You seem to have a spouse problem more than a child problem. Kids are always looking for power and control, and apparently she found a route--complain to Mommy who then complains to Daddy.

Your DH needs to shut that down. Her Mom has no say in your household so your DH needn't even respond to her or take any action based on Mommy's opinion. And your SD should hear loud and clear from him that if she has a problem, she goes to HIM, not Mommy.

A spouse does not get "in trouble." If there is a disagreement, it is solved between the two adults. Your spouse is treating you like the child and his daughter as the authority. Just NO. HE 100% MUST TREAT YOU LIKE THE LIFE PARTNER THAT YOU ARE.

Marrying was not a terrible idea. What is a terrible idea is letting a minor child terrorize you, your mate, your marriage, your home, and your family.  Your DH's X has shit for nothing to do with you, your mate, your home, your family, or your marriage so stop cowering to your DH's baggage, stand  up, give your DH clarity that this coddling of his failed family is done and he needs to step up and  be your equity life partner and stop making  you second or more accurately last, to his failures as a mate and a father.  You did not agree to an equity life partnership where you are the only partner who is all in while your DH is all in only with his past and his baggage.

This is a critical success factor in any blended family marriage. To not have it is to ultimately fail in yet another marriage and to fail yet again in life, love, and family.

The only thing and the only one getting your DH in trouble is himself and his lact of testiclular fortitude in manning up, parenting with authority, and putting his X in their not part of his life and family place. Divorce ends a marriage and it also ends a family. That family must metamorphose into something else that does not include Xs being partners other than to comply with the CO or if possible collaborate reasonably in raising their kids. If that is not possible, the the X stays the hell out of your lifes and she and your DH can parallel parent rather than coparent.

It is time for your DH to put a hand between his legs, grab a big hand full of man sack, man up, be YOUR partner and keep his baggage in their place rather than demanding that you continually sacrifice yourself on the alter of SParental martyrdom to him, his X, and their failed family progeny. It is long past time for your DH to firmly put his X in her place and apply some discipline to his spawn and make it clear to her that her mother has zero say in what happens in his mome and that if his spawn runs to mommy to whine and cry that things will become far more firm in her life at daddy's.  Be ready for the SKid to try to not come per the visitation schedule. Daddy needs to smack mommy with a contempt motion every time she fails to surrender the SKid per the schedule. Minors do not command their parents.  Time for this one to learn that lesson firmly.

Set the standards of performance and standards of behavior in your life, home, family, and marriage and enforce those boundaries without exception. This is a proverbial hill to die on in the blended marriage/family world.  When you married in addition to being equity life partners you and your SO also became equity parents to any children in your home and marriage regardless of who spawned those kids.  My bride would have tolerated nothing less from me as her partner and as a parent to her son.  Neither would I.

30 years later we have raised a man of character together, lived a life of adventure, and built a love for the ages demonstrating what being viable adults, committed partners, and standards based parents looks like.

If this guy is not that guy for you, cut your losses and move on.  Far too many SParents jump in and spend far too long polishing a turd that never becomes the diamond they hope to create.  Far too often at best they get nothing but a shiny POS that sadly quickly tarnishes when the continual shining and spritzing with perfume wears out the SParent Spouse.  It is far better to actually go forth, live life, and smell the roses than wonder why the shiney turd smells like the shit that it is.

Be good to you.

Live your best life. If you don't no one can or will do that for you.

IMHO of course.

Good luck.