I understand your feelings,
I don't know what the point of getting married now would be?

Plus there may well be bills and expenses after all is done.
You as the wife would be responsible, I believe.
I'm not a lawyer though.
I guess you have to look at the legal as well as the emotional side of it.

My bil said some unusual things near the end, he wasn't taking any pain meds - but that could apply both ways.
Meds could cause some thoughts to be voiced as well as the terminal cancer pain.
Is he lucid most of the time?

But it is a nice gesture from him.
If he brings it up again I would say "that is very sweet and thank you". And just leave it at that.