SD18 steals, she steaks from school, her baby brother, her mom, us.

She stalks money mostly, but also food.  She stole a bag of her step dads' door gummy worms (like the giant bulk bag), she's stolen her brother's birthday cake (eaten the entire cake within 1hr), she's stolen over $100 worth of fundraiser candy from school, then stole from her mom to pay for it, then some more fundraiser candy, then stole money from her baby brother (birthday money from his piggy bank) to pay for it.  

So yes, yes it is common

Regularly steals. She would cruise Toxic Troll apartment in the wee hours cruising for items she wanted - clothes, cash and even some checks that she kited from her mother, using her child account as one of the kiting places. This caused TT to be locked out of her bank and almost caused Husband to be locked out too.

I wont live with her ever again because that would just really put me over the edge.

Do you have any web security cameras? Consider getting them. I am.

DH's youngest daughter stole our wedding cash gifts....over $200. She was the ONLY one who could have taken that money. She also stole makeup and other small things. And DHs youngest son would "borrow" video games from my son's collection. What a damn nightmare. So glad those days are over.

EDIT: Different kind of theft. We LOANED DHs 2nd son $7500 in three different loans. Had countless emails and texts about when we were to be paid back, where was the payment, what's our address, etc. over the years. Then DH retired and demanded he be paid back. Said son then says, you never paid me back for my overseas driver license. You owe me $10,000, which meant DH should pave to pay him an additional $2500 over the loan amount. DH did research, found out true price of license and deducted that amount from the loan. SMH

It is to be noted we would NEVER loan that man anything regardless of his situation. He could be homeless and we would NOT give a dime due to that BS move. DH did NOT owe him anything. Damn liar and thief.

My SD61 is a thief who has stolen from us for years and steals wherever she goes.  That may be why none of her siblings want her in their homes.  She seldom keeps a friend, probably for the same reason.

I maintain a grayrock relationship with her for DH's sake but hide my purse and any valuables or sensitive paperwork when she's coming over.  Altho DH is in denial, he hides his wallet, too.

She must have done the same with BM and her husband, Clueless.  BM died, then later, Clueless was sick with cancer.  He told his brother about her antics so the brother alerted the neighbors to call the police if they saw her car.  We all know how she could strip a house.  She wasnt allowed at his funeral, either.

Altho her preferred items are money, drugs, jewelry and valuables, she is partial to pretty things, a crystal glass, for instance. Anything she can put in her purse.

Make this POS sweat.

First $2K.  It was supposed to be paid back in $250/qrtr increments over two years.  She wrote one Check. Then called and asked us not to cash it.... blah, blah, blah.

A couple of years later she called DW crying, her DH was abusive, she needed $750 for a deposit on an apartment so she could move herself and their boys out of the marital home.  We did not give her cash. We paid the deposit to the apartment complex directly.  She did not sign the lease, got the money refunded directly to her, then went on a beach vacation with her HS BFFs.

Never again.  This cousin is a Unicorn because she was the closest relative/relationship my DW had with her paternal family. DW's father was killed before my MIL found out she was pregnant with my DW. So Unicorn is a mythical and magical element of my DW's childhood and they were each other's person for much of their lives.

Unicorn cousin passed about 8mos ago after a very long illness.  

My DW avoids the memory of the Unicorn stealing from us.

SS stole from 14-19 (he's 19 now and I know it will continue for years) and would manipulate my OH into house sitting too. He'd say he was doing it as a favour, wasn't he good boy etc, and say he feels so much cosier there than at his mother's house, appealing to the play-off rivalry between them too. He stole money, presents that I had hidden away, my toiletries, and bare-faced lie when asked about it. When he would stay over 50% of the time, he'd get up after we had gone to bed and sneak around the house stealing. I know because I I caught him. He would also do some other non-theft but really bizarre stuff after we had gone to bed.

It's terrible your SD stole, even your brother's valuable painting? Have you looked on websites to see if she's sold it?  Can't you both tell her that she has one last chance to admit it, or - especially because of the valuable painting - you're going to have to get the police involved. And see what she does then?

In case you need to give yourself permission to stop the madness,,,

Would you allow  thieves from your community come into your home and live there?

I am sorry you have gone thru this, we have too.

Difference is, they are not allowed inside our home, ever again.

Not from us, but from DH just after we started dating.

DH and BM were divorced when the  skids were older teens. One had graduated high school, the other was a senior at the time of the divorce. The skids moved in with BM, but still had keys to the house where DH lived. Then a couple of years later, BM died. DH wanted them to move in with him, but the skids decided they'd move in with their boyfriends. 
DH comes home from work one day to find that oldest daughter and boyfriend had been in the house and removed items, including the bed he had been sleeping in.

DH was furious. He called his child and told her that he wanted the items back. He told her that he'd file a police report if they were not returned (and that that would mean charging her in addition to her boyfriend). Two days later when he came home from work,  the items were in the garage. They weren't in the house because he had had the locks changed.

DH is a marshmallow in a lot of ways, but taking the bed pushed him over the edge.

Back when my DH and I were going through the trials and tribulations of BPD teen YSD living with us, a private joke we shared was "At least she doesn't steal." Her constant lying was awful, but a thief is the top of the scum from the bottom of the barrel. Stealing from family is the absolute worst.

Satan and Killjoy are both compulsive thieves. It seems to stem primarily from hostility, the greed is secondary. DH could leave his wallet with cash out on the kitchen counter, but Killjoy only stole from me, things she couldn't even use, like shoes that didn't fit her.

Both are also compulsive liars. Look up "duping delight", it's a feature of sociopathy. They get off on lying, and get an adrenaline rush out of stealing. Adrenaline junkies are addicted to adrenaline.