I'd move out. id hate to be raising my baby in that cramped a place and with my mother's baby too. i'd find a way to make it work, it'd just be way too hard to stay and it ain't fair on your brother and sister to expect them to share.
my mum and i were pregnant at the same time my sister is 2 weeks younger than my son
It's really not a huge deal for her brother and sister to share a room. Families do it all the time. I've had to with BOTH my brothers at different points in time. One is 8 years younger than me and the other is 2 years older. It's not the end of the world and like she said, it's not forever. I understand it's ideal to have their own rooms but this particular situation I believe its best they just share, UNLESS they get a sofa bed. THAT would be most ideal.
Two newborns, a mother and daughter in the same medium sized room would be way to hectic and crowed. There is furniture and such ya know. And I think her siblings would much rather share with each other or one of them sleep on a sofa bed than be in the same room with a baby.
ETA: I don't think there is anything wrong with you holding out on moving. You don't want to be under anymore stress than you have to be. And they're your family, they should support you and understand that a family is a united system. Everyone has to budge and bend sometimes. x