To expand a bit on what I said in the first sentence of my earlier reply, it's not clear how much of the house was a gift and how much was inherited. You said that your mother-in-law added your names to the deed. You didn't say that she removed her own name. If she added your names but did not remove herself from the deed, then she remained a part owner, and she only gifted you a partial interest. In that case, one of the things you need a lawyer to sort out is what each owner's share was. The possibility of a life estate, which makes the gift incomplete, adds another level of complication.
If your mother-in-law was still a part owner of the property at the time of her passing, then someone inherited her share. That is not determined by the deed. It's determined by her will, or by state law if she had no will. And the basis of an inherited share of ownership is different from the basis of a gifted share.
If she did remove herself as an owner and deeded the entire property to you, you still have to deal with the life estate question. And since you say "we," "us," and "names," it's clear that there are multiple owners involved. An examination of the relevant documents is needed to determine what each owner's share is.
