It is very wise to plan ahead abouthow your mother wishes her inheritance to really be used.  You should speak with an estate planner and/or an attorney in your area on the matter.  Your Mother can create a trust that names a particular trustee there than your daughter's Mother.  She can set parameters on the trust - what it is to be used for and how it can be accessed - but those parameters will follow what is permissible under your state law.  She can also have the trust dissolve at 18 when your daughter becomes of legal age to cotrol her own money or at what ever age she chooses to elect.  She can and should name a successor trustee in case the trustee becomes incapable to serve.    If your daughter's Mother can access the funds it will depend on what the terms of the trust have indicated and again, that will be according to your wishes and state law.  Good luck.