1. The undivided joint property can be sold by the co-sharer his own sahre. There is no legal impediment in it.
2.So both you and your mother can sell your respective shares leaving your brother's share intact.
3.Another option is to declare your brother mentally unfit and then get your mother declared his guardian and then sell his share as well with the permission of court.
4. So if you adopt the third option advised above all the problems resolves.