I think for Bella and Rachael, who just went through the whole ordeal of having to accept each other, to accept Liza too would be pushing things too far if the MC was the one who revealed it.
In my mind, the ideal situation would be for the MC to "catch" Liza and Bella together. With Liza being in on it of course. Bella would try to explain, but Liza and the MC would just tell her not to worry about it. She wouldn't ideally have a foot to stand on once the truth was revealed at that point.
Then Bella would be able to help with Rachael, probably by having the 3 sisters meet together alone once.
As far as Rachael and the mother situation, I can foresee Susan's "Mistress" persona being used to take control of Rachael and the MC's time together.