It depends upon how she has the house deeded when she adds you as co-owner. If she adds you as joint tenant in common, when she dies, her half of the house will have to go through probate.
If she has the house deeded with you and her as joint tenants with rights of survivorship, then you will own the entire house when she dies.
There may be tax issues with her adding you as co-owner, if you end up selling the house in the future.
In addition, if she needs to go in a nursing home, it is possible that the value of the house will need to be used to pay for her nursing home care instead of medicaid paying.