Wow - thank you all so much for your support and best wishes. Of course this is all very early in the process, but somehow even writing this down makes the dream that much closer.
We have tried to get my husband's family to help with MIL such as having her over for a weekend (or come to our house for the weekend) so that DH and I can have some quality time together but it's always "we're busy" from all of them. We have come to accept the fact that we are very much alone in caring for MIL at this stage so at the very least, SIL can get moving on her end of the bargain with the assisted living hunt.
I certainly hope I don't sound like some ungrateful daughter-in-law. I think the past few years of caring for my terminally ill FIL as much as we could by going to their home every day to assist, then having MIL move in with us when we realized she was not coping well on her own in the house, and for the past several months also paying my Dad's rent at his assisted living facility in another province - well, it's just gotten to be a lot to handle.
DS is totally on board with us selling the house and we would certainly look at getting either a two bedroom or a bedroom and den so that he can live with us, at least until he's on his own two feet. He thinks it sounds exciting.
There will be much to do in terms of getting MIL situated and moved and doing some work to our house before it is ready to put up for sale. As I said, we're probably two years minimum away from realizing this change. But it feels good to look ahead.
Mochi - thank you SO MUCH for your rousing endorsement! I love hearing how this sort of change was so incredibly positive for you.