Be ready for the SK's to use this as a way of manipulating their dad. See she hates us, she only wants her baby around. Crap like that. Truthfully, I wouldn't keep them from seeing their sib, but I would enforce the no social media devices unless I say rule, and would speak to DH about that now. And if he pulls the "It's fine I do have two other kids" shut that down now. He may have two other kids, but you're obviously not thrilled with how they are turning out. Plus this isn't just his third child, it's your first and you have every right to set boundaries you are comfortable with. And simply stateing the fact, you don't want me to tell you how to raise your kids, don't tell me how I should parent mine.
I didn't leave my kids with sitters ever unless it was my mother. Teens today have no sense of responsiblity so not wanting to leave your baby with two teens is perfectly understandable and you just have to be upfront and honest with DH about that. No sugar coating.

I wouldn't use the word demand unless it's the only word left. But I would explain to DH that he and the baby BM might be comfortable with all the picture sharing and what not but you are not. And an infant really can't say if he or she wants their picture all over the internet so while around the baby leave the cells, iPads, or what ever in their rooms. Perhaps use this to teach the SK's some responsibility by involving them with the protection of the new baby. You're trusting them to protect their sib from any harmful eyes on the internet and what not. And if it actually comes to it, put the foot down. Ok, you can't respect my wishes, go away then. What is all comes down to is they are not your kids, you have to do what is best for your kid.

I would err on the side of engaging the SKids in the new brother/sister experience rather than going for a position of dictatorial control. This is also your DH's child and though not his first this is his first with you and the first for you and him together. DH has equal say on the new baby. I think you would be well served to take the time to firm up the equity partner/parent bond with your DH before the baby arrives. This whole experience will be much more positive if you are a team rather than struggling against each other.

Like it or not, your baby will also be your Skid’s younger sibling. Engaging and managing their interface and relationship with your baby will be far easier than battling the Skids to control how they interface with their little sib.

At some point you will want a date night with your DH. If your Skids are responsible then I see no issue with a 14yo sib watching their baby sib for a few hours while you and DH enjoy an evening out. If the SKids are not trustworthy due to their immaturity then find a sitter you are comfortable with. When I was in my pre and early teens my parents would hire a teen girl from up the street to watch my little brother. I am 6 years older so not quite enough older to watch my 3-7yo little brother at that time. I did not have an issue with the 15ishyo sitter watching my little brother, or myself for that matter. Hey, I learned a lot playing strip poker and spin the bottle from Val from up the street when I was in my early teens. }:)

With the social media thing I completely concur that you and DH should be the ones to maintain control over posting images of your child. This is easily handled by turning off the internet router and cell phone service if the Skids do not comply with the rules that you and your DH set on this. Talk with the Skids, tell them that until you and DH are comfortable that neither of you want pics of the baby floating through the electrons.

IMHO of course.

Congratulations on the new little one.

I wouldn't let a stranger treat me the way they do. I'm so worried about my baby seeing people treat me this way in my own home.

That's what you tell DH. Let him know that this isn't gong to work for you. Even if his kids have to fake it when around the sib then they need to fake it. And remind him how you would never disrespect him infront of his kids.