Plan ahead
IANAL. Start by assuming that the will is going to be contested when she passes away. Then get a lawyer and provide evidence of her state of mind.
If your brother is OK with all of this right now, then get him to sign as a witness to her will and have him sign that she "knows what she is doing" (written in appropriate legal speak).
Then, have her explain her reason for making the change on videotape. Do this when she is of clear mind. If she can convincingly explain why she is making this change without appearing forgetful, confused or coerced, then you should have enough evidence to rebut any suggestion later on that she was incapacitated.