Are we liable for my mother-in-law's future expenses?
Your husband is not responsible financially for his mother's needs although obviously you and your husband have concerns for her welfare. A document, such as you are suggesting has no legal significance, but also is not needed to insulate yourself from financial liability. If you and your husband are concerned that her funds are being used inappropriately by your brother-in-law, you should consider filing a petition to be appointed as your mother-in-law's guardian in order to manage her funds in a more efficient manner.