Black mail sister into taking photos since you slept with her and she is married. Something like

Pose for these or I tell everyone about your (mole, frekles,tatoo, how she shaves) and how I know about it. Then post in the VIP room. Let me know if you need online storage for the pics. lol

Oh and keep sleeping with her. Make the black mail a joke at first. Married women are freaks to thier lovers. They will do anything almost with the man not thier husband. She might just take the pics without the uglyness. And if you want to drop the bomb...

Christmass dinner would be a good place. YOu have months to set it up so everyone will be together. While passing the potatos you let it out and leave the whole family to fight.

Husband, sisters, mom and dad.

Something like,

You know I have something I have to say. I slept with both Insert names here. I have decided this is too much for me. I just have to go. I am sorry.
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