I would have a very different answer if you were very young, but at 25 I would confront your mother alone directly. Let her know that you are aware and that it is too difficult for you to keep this secret. Talk only about your own feelings — not about your mother’s behavior. You are the world’s expert on your feelings and you have a right not to be put it the situation where you have to keep a secret from your dad.
I know you say you don’t want to break up the family, but if you found this out how hard is it going to be for your father to find out? These things typically are only a matter of time before they blow up. Then you’ll be in the predicament of having known and kept it from your dad.
This isn’t about saving the family now — this is about taking good care of yourself. Don’t try to script this for your mom — don’t tell her what to do or not do. Just be clear that you having to keep a secret any longer won’t work, and she has to do something because you won’t keep this secret from your dad. This isn’t a threat. This is what you know to be true for you. If this continues the pressure on you will continue.
This is a very difficult situation but I would be as honest as you can with your mom and put the ball in her court. Set a timeline for her and don’t be afraid to remind her. She may not like it, but she’s put you in a terrible position and now it is time for her to deal with that reality.
As you get ready to do this I would highly recommend getting some individual psychotherapy. You may want to get this support immediately so that you have someplace to go as you deal with your mom.
Wishing you patience and peace,
Proof Positive Blog @ PsychCentral