My personal experience and opinion says that- it really doesn't matter whose statement. at USCIS, they don't care now since she is already here. The only thing they will care is what is the basis for requesting extension. WHat reasons? Why your mother need extensions - show that with valid and enough supporting documents. For Example if you or your bro is getting married soon, or you guys having baby soon. Just becuase she need extension or she wantsto stay more is not a valid reason.
But guess what, If you file extension before her i-94 expires, she can stay as long as USCIS takes time to make decisions. and they usually take time in b1\b2 extensions. So my suggestions is that when her i-94 is about to expire, keep 4 weeks of time period, and file extension. In worst case scnerio, if her extension is denied, by the time she receives her letter it will be another 3-4 months.