OP,
I can well understand your frustration.
Some of the responses here are so utterly deluded.
Of course this is going to be a massive transition for your family and you have every right not to want to go back to the baby stage under your roof.
Perhaps you will be lucky and your SD will suddenly morph into a responsible mother at 19, or she could just expect you all to suck it up and step in and become default parents.
For every happy ending their are also many, many parents who ended up raising their grandchild, while the young mum's want to carry on with the socialising that people her age should be doing.
It is very hard to remain separate when you see a young mum not engaging.
Will she expect on tap babysitting from you all?
I think you need to have a VERY frank conversation with your wife away from home, on what she thinks is going to happen.
The idea that you have no say in a house that you share is not acceptable.
The idea that your wife thinks you have to just suck it up, is not acceptable.
I think a VERY frank conversation needs to be had with your SD by her mother laying out some boundaries.
You also need to be part of the conversation.
If you contribute to the cost of the house, you should have a say.
A new baby WILL change the dynamic of the house.
Your young teens may love it, they also may not.
Nothing is a given here.
Resentment is a huge possibility here if you can't speak frankly.
The idea that your wife can assume the baby will stay in your home without so much as a conversation with how you feel would make me think that you do not have a very healthy relationship at all.
In your place I would be very pissed off not to be consulted.