Thanks people

Sometime early this year i began to suspect my older sister was having an affair,with her husband's friend who stayed with them while on posting to the town they live in. Now im thinking its not just him but as much as two or three others. There is every chance that i might be wrong and she is just being flirty with them-this is usually her husband's stock in trade with other women. Bt she has began to disrespect him,openly for that matter and this is a woman that worshipped him before,that could catch a cold if he sneezed! Could she be doing this to make him feel her hurt as she has tried everything possible to let him know he hurts her. My question is what do i do? Act like its none of my business,drop hints that im aware with subtle warnings or directly confront her?