Don't become a co-owner unless your wife is an only child. Other siblings may see it as an inheritance grab.
Secondly, it would have tax implications for *you*, as you would then have your MIL's "basis" in the house. If she bought it awhile ago, if/when she passes and you sell the house, capital gains would be based on the delta between what SHE paid, vs. what YOU sell it for. That could have a serious tax bills associated with it.