Should trust distribution shares determine the Trustees?

My wife's mother recently changed her trust distribution to give my wife and two brothers an 81% share of the distribution. All of her siblings were informed of this and did not object.

The current co-trustees have been so designated for the last 15 years. They are a different brother and a sister who between them now only have a total of 4% of the distribution. This brother and sister have an okay relationship with my wife but have had ongoing (20 year) feuds with the two brothers who share the 81% distribution. This seems like a perfect recipe for a total disaster when her mother passes.

We have tried to convince her mother that she at least needs to make my wife a co-trustee because she can bridge the two factions. The two brothers who share the 81% distribution also desperately want her to be at least co-trustee if not the only trustee.

What guidance would you give my wife?