Why do you need to mention the lady in question? Her memories are all frought with a lot of negativity and drama right now. The bottom line is you want to offer comfort to your SiL... (Is it your Son-in-law or Sister-in-law?) By all means make a quilt or what ever you want to make. Just make sure it is offered as an object of comfort.....NOT to mention Mother...the emphasis now should be on the survivor(s)... so give your gift for his or her comfort, not with sad strings attached.
I say this from the point of view of someone who is in the middle of five generations of severe depression. It is seldom really understood by others. The deceased victim should not be blamed, but usually is.