moomin you poor thing. That's a horrid situation to be in, with the sort of disbelieving premature grieving that's going on, they're all going to still be quite angry at the whole situation. If you really feel there are issues beyond just the "look after him, he's sad" nonsense they've heaped upon you (and tbh from what you've said it sounds like the mum and sis ARE both being epic bitches about the whole preggo shebang) then I say damn well have it out with them. If you feel you're up to that, that is! Maybe being concerned for the family and wanting to visit them to see if you can be of any support might make them more keen on you, if not then you are absolutely within your rights to tell them how you're feeling. If they want anything to do with your baby then they are damn well going to have to deal with you! And none of that being nice to your face rubbish, you'll know they're not sincere and frankly it's an insult to be lied to, I've had that with exes' families. Ugh.
Sorry for ranting, everyone - I have BIG issues with DH's stepdad. DH's dad left him and MIL when DH was about 3, and stepdad came in when DH was about 9/10, and is still around. He's bigoted and ignorant and blinkered and intrusive and nosey and just damned irritating, but he means well most of the time. I have strong feelings about the baby not referring to him as "granddad" or "grandpa" or any of that, he is NOT the kid's relative and DH never called him dad so baby won't either. DH's dad passed away just as he and I started dating, it was horrid and I wish to god I could have known him better than the one phonecall we had. So Bean will know stepdad by name, and there will be a picture of their REAL granddad so they can see him and ask about him maybe.
In-laws are my pet peeve
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But seriously moom I'd be brave and bloody well find out why they treat you so badly. They might not even know they're doing it, maybe? That'd be a blessing!