My mom's cheating on my dad. I have caught her several times talking to men, rather a man, in a flirty way. She makes plans to go out and spend some time with him. For this she lies to us and dad of course. I had my doubts but yesterday I eavesdropped her talking to that man. They were talking about love and all. I could not bring myself to discuss this with someone I know although I did discuss this with my elder sister. She wants me to confront mom directly but I can not do that because after that we would never have the same relationship and respect.
Moreover I don't want anyone to lose respect for my mom but it seems my sis already has lost it. I am in an utter state of confusion and don't know what to do.
I feel bad for my dad because he is too good and I just don't know what to do about all this.

Is your older sis somewhere far away? Because ideally you would talk to mom together, in private. If not, I suggest that you work on what to say with your sis and then confront mom in a quiet, non 'confrontational' way. Just for your peace of mind, not because it's your burden to protect your dad. It isn't. He may know, he may not want to know, it may be something you have no idea about, such as he had a romance with another woman, or they plan to divorce. Spouses need to work out whatever goes on between them with each other.
Just tell her what you have seen and heard on about 2 or 3 occasions and tell her it hurts, but try to keep anger out of your voice. Be prepared for ANY confusing response, from denial, to 'none of your business', to walking away. Hopefully if you are tender about it she will talk. The fact that she hasn't hidden it very well is an indication that she isn't really trying to be secretive, and may be glad to talk to you.

Wow... this is tough. I'm sorry that you are in this position... no easy answers for how to approach this subject. Do you want to talk to a counselor for advice on this one? Some outside perspective might help you to decide what you can/should do. Anyway, if you'd like, you can call Focus on the Family's counseling dept. 855-771-4357. They offer a free phone counseling session... just a thought. I work for Focus and know firsthand that the counselors there are helpful. I hope your family can get some good answers/advice... keep us posted, OK?