tricky situation.
where do you anticipate the rift? between you and your brother?
if so, i'd just speak to him and explain that you'd really rather not having more to do with SM1 so please don't report back on any aspects of his continuing relationship with her.
offer to do the same wrt to your having a good relationship with sm2.
if you're concerned about a rift between your brother and father then i'm not really sure you can do anything beyond talking to them both and suggesting any solutions that might be there for whatever issues they're having. if they're hell bent on falling out and won't listen to reason then i'd advise not getting drawin into it.
same thing, talk to both and tell them you won't be taking sides and you want to continue a good relationship with them both.
fwiw, my relationship with my SM is very sporadic as my father died when i was 17 and we live at different ends of the country.
i am a bit sad about it as i really like her and her kids. we all have kids of our own now and i wish we could all get together a bit more frequently and keep in touch better.
my SM always sends cards and gifts at xmas and we talk once or twice a year on the phone but she remains distant otherwise.
i think she finds it hard as she and my father were immensley in love and he died very suddenly.
sorry, i've digressed a bit!
hope things work out okay for you.