I can only add to why you don't want to do this. There's no need to 'answer' my questions on the Forum. They're just for you, et al., to think about before you begin.
Will the 'change' make the house 'out of character' in the neighborhood. (Or do you live on a large, wooded lot where it doesn't really matter what the house looks like. In which case, there's no 'out-of-character' issue ...) Will you have to upgrade your electrical service into the house? Will you have to upgrade your HVAC unit to handle the added 'space'? If you have septic, will the current system handle the additional load? Will your neighbors object because you're 'blocking' a view? It's possible the existing framework won't be able to hold the new 'load'?
IF you're still considering this, and are serious, it may cost you some $$$. Why? This is not a simple remodel job or such. A professional would have to spend some considerable time with you to go over your plans. And time is $$$. In fact, you may need an Architect to help w/the planning. Soooo, if you're serious, plan on spending some $$$ BEFORE roofs are lifted. Plus, it would mean that all of you would be 'put out' for quite some time so the work could be done. Don't think you could live there while the roof is off.
I know I and 'others' could go on but I think you get the picture. I too think you're better off in getting your own place. WHat you could do is consider getting a place very close to the house so, if you're thinking of taking care of your in-laws, you're a short walk away (not a short drive.) That may make them feel better.
I think I've said enough. My best to ya and hope this helps.