Trying to integrate this post with your first one, where you said you told the kids you were Dad's wife, not a mom replacement, and that  you and Dad lived overseas for many years you two lived overseas and had minimal contact with his kids.  I think given that, your SS's actions are not unreasonable. 

I do not know if they have picked up on your secret desire that you would like to be primary caretaker for their child.  They may know that you are unhappy about not having your own kdis. That would be VERY, VERY scary to me as a new mom, if I thought that you coveted my child.  

You are not your SS’s mother, therefore, you are not his children’s grandmother. He owes you nothing except the basic respect everyone is owed, IMO. 

As long as SS is respectful to you in person, don’t push this and DH shouldn’t either.  

It isn't your decision or right to decide when they should tell and in which order. For my pregnancies, I only told those who I thought I'd need if I ever had a miscarriage. Asking is rude and probably put them in a weird position. Are you sure they don't think of you and DH as a unit and that's why they told only you? There are plenty of things I tell just my mom, but I expect she brings it to my dad. 

Yeah, I have to agree with what has already been said by others. 

Don't spoil this for your SS and his wife.This is a very happy time for them, as it should be. You can't live your life through the life of others. While I'm not unsympathetic to your desire to have always had a child of your own, your "secret wish" that this young man and his wife would give you their child to raise? You've had 20yrs to have found ways or your Dh and you to have brought a baby into your household. Don't blame your SS or try and make him and his wife feel guilty over their first child.

And yes,I find it pretty odd to hold yourself nothing more than 'Dad's wife' all these years , to now suddenly expect to be treated as if you are the biological grandparent due all  'rights'  and find it somehow disrespectful they did not race to tell you about the forth coming child first with due congratulations. 

My suggestion is to a professional therapist (with experience in stepfamilies) and try to sort your feelings and put them into perspective. Several totally different things occurring here that do intertwine but each with it's own significance in role.

It's a little too soon to be upset. You weren't home when SS called. You, yourself, said that you don't know how the conversation went. They didn't wait to call when you weren't home. They told DH. DH told you. Don't look for things to be mad at. 

When SS and his wife tell people about the pregnancy is their decision. Yes, she most likely told her mom right away. BM may have known before DH and you. But, don't make this a contest or you will definitely lose. 

They told DH. DH told you. Ball in your court.  Rise above your feelings and tell them how happy you are for them. And then step back. Realistically, step mothers, who did not raise the children, are often low on their priority list. You need to accept that your relationship with the SGC is contingent to DH's relationship with his child.

I think everyone pretty much covered it. You never acted as a mother to the kids, which means they may not treat you as a grandmother. That is their right, and you and DH need to accept that. If your DH runs in there demanding you get equal grandparent status, he may find himself not being a very involved grandpa.

Also, if SDIL has suffered miscarriages or issues getting pregnant, or had an early pregnancy scare, she may have waited longer than usual to tell folks about the pregnancy so that she didn't have to tell all those people again if something bad happened. Especially people who only see themselves as "friends" because they happen to be married to a bioparent. Or, they just didn't think you guys were a priority to tell. Again, their decision.

Overall, sit back and let it all unfold. Unless they are overtly disrespectful, you and DH have little to address with them, and any addressing may just result in less communication.

Be happy for them. This is THEIR child.

I did not receive  an  email, text or call to congratulate me on soon becoming a grandma.  

I find it very odd that you expected this.

Do you realize that this could be your big opportunity to be more involved in their lives? As you were absent when the call came, this is the perfect opportunity to call THEM and tell them how absolutely thrilled you are for them and to remind them that you are there, ready and willing, if they need anything. 

I know that you probably don't feel at all like doing that but think how much potential good it could do for your relationship with them. You want them to think about YOU not about dad's wife, right?

My grandparents on my mom's side were divorced when I grew up and each had significant others in their lives. My grandmother outlived 2 husbands and my grandfather was with Carol as long as I can remember. I never called them grandma/grandpa they were always called by their first name but they were always involved as much as my actual grandparents and I never thought anything of it. As a child I didn't really ever think about their relationship to me. They were always important people in my life and that's all that matters to a child.

If your true goal is to have a relationship with this child don't worry about being offended right now. I wouldn't expect my SD to treat me the same as her mom in this situation it'&s just unrealistic. If you have a good relationship with SS then I'm sure you'll have a place in the child's life. 

I honestly find it absurd that you expect this and find it incredibly selfish and find your approach incredibly narcissistic to be honest.  YOU are to call and congratulate THEM on becoming parents/being pregant.  In what world is this about you???  You convey you were an uninvolved step-parent....and expect a 2nd call to congratulate YOU?   I can see w/good reason you may find them keeping some distance between you and the child. I sure as hell would.  If a stranger on the internet sees through your behavior i'm sure your SS and his wife do too.  Your dh's expectation is pretty absurd too.  This is about their pregnancy and starting a family.  Don't push/force yourself on them.  Call them with a sincere congratulations and you are happy for them and there for them if they need you and that you are thrilled to meet the new little addition.  Shows you are excited want to be a part but also respects it's their child/decision.