I can read from your post hiw much these two issues are affecting you.

I don't really know what to advise, other than to second what Tashybabe said.
Have your family had a chance to debrief and/or have counselling in regards to your nephew's stillbirth? From what you wrote, it sounds like they are still grieving and any baby (yours or anyone else's) reminds them of their loss. This is not your or your baby's fault or your problem. They need to make peace with their own feelings about what happened. How close are you with your sister and your family? Would they be willing to try some counselling with you?

Re you SIL - I've read your previous posts about here and she sounds horrid. What do the rest of her/your DH's family think of her treatment of you? If they allow it and don't see a problem, then perhaps you need to distance yourself from them all.

A lot easier said than done, but please don't let others dump their issues on you or make them yours. Neither of the things you wrote about are your issues, as you have no control over what others think or do. I'm sure that no matter what happens you will have a wonderful birth experience and you and your baby will be surrounded by love and positivity.