I would rather my brother not fuck my partner, but maybe that is just me. I'm very sex positive, but I expect to hang with my family for gatherings at Christmas etc. You have obviously figured out whether you can continue a healthy sexual relationship with your sister in law, with or without your brother, or whether you want to damage your relationship with your brother, or you just don't care.
Your bro already has his legs cut off, and a woman who is sticking with him. Here is the way I see it: (a) You steal his wife. (b) You fuck with family dynamics. (c) You decide to fuck her for a while and then have to deal with her at family functions. (d) You fuck her and destroy the relationship with your brother. (e) you decide on alternate nights with your bro on when you fuck her, and share her for the rest of your life.
It's done now, so there is no going back (and if it is a trolll ..... rrrrrr). I really think I would spend a lot of money on making my wheelchair bound bro happy rather than fucking my sister in law.
I'm prepared to concede that you may be doing the right thing. I don't buy it though, and I would prefer to rent prostitutes for my bro than doing what you are doing.
But maybe, I'm just a bitch.