Here's the thing: You don't have to be excited about other people's pregnancy, especially when you have your own to be thinking about and enjoying. *shrug* I'm due within 6 weeks of my sister and SIL, and, honestly, I was a little disappointed when I learned they were pregnant and due so close to me. They can't really help when they're due, though, so it's just one of those things you have to accept and move on from. I'm happy for my sister and SIL, of course (especially SIL as this is their first), but I'm not going to be doing cartwheels for them any time soon. We chat about pregnancies, but I don't go out of my way to ask how they're doing, etc.
As for the housing situation, I think it's another thing you're allowed to feel upset about, to a point, but then you need to accept it and move on. It's temporary, and moving in with parents will always have a downside. The circumstances surrounding your SIL's pregnancy don't change the fact that she is pregnant and due shortly after you and needs MIL's help. As for expecting you to help babysit, I think it's reasonable since you will be there to pitch in here and tere, but I would also ask MIL to not rely on you to do the babysitting yourself. Have days where you're willing to help, and days set aside for you when you will only help in an extreme case (2-3 times a week), days that you would plan to get out of the house, run errands, and just... get out. You may not get out on those days, but those days she should plan on doing the babysitting herself, since she is the one that volunteered to do so.