Don't lower yourself to the way others are treating you. You are now going to be a Mum and need to take care of the baby. If you stay the mature one and just ignore their behavior then they will soon cut out the childish behavior and no doubt come to you when they have settled down and all will be fine. I agree that age has nothing to do with maturity alot of the time, some women at the age of 22 act as though they are 30 but others still act like they are 16. You act and live they way you chose to.

Your sisters sound like they are only 16

Ignore it, move on and keep yourself well and things should come good. In the end they are family and they will soon come around and be happy for you.