SIL keeps 1/2 of home equity that belongs to her mother. (CA)

SIL, MIL, Wife, Me all live in one house. SIL and Wife are both named to the house. Everyone pays 1/4 of the bills for the house.

Fastforward and they end up selling the house where me and my wife end up with 1/2 meanwhile SIL ends up with the other half. SIL decides to keep the 1/2 instead of paying MIL(her mother) what can we do in this case?

SIL mostly pays 1/4 in cash to my wife who pays the house under her account.

No writen agreement that SIL would give MIL as we all agreements are spoken. We never saw that she would do this to her own mother.

Northern California