Mom had an affair - having a hard time feeling bad

So a couple of years ago (November 2013) my mom, dad, and grandparents went to Israel for my mom's 50th birthday. They had a tour guide there that my mom stayed in touch with. I knew they had a tour guide there, but I didn't think anything of it because that's pretty normal in Israel. Come August 2014, my two siblings and I get a message from this random woman from Israel saying that my mom is a home wrecker and all this stuff about her. I thought this woman was just crazy, so I told my mom about her. Turns out that this woman was the Israel tour guide's S/O, and my mom has had a physical and emotional affair with this guy. My parents' marriage was already in the fritz, and I think this is the straw that broke the camel's back in my parents' relationship. I don't think my dad was happy about it, but he was not angry. My mom moved out of the house at the beginning of September 2014. For the next few months after, I had assumed the whole thing with the tour guide was over because she was caught. Nope, not the case. My mom has continued to keep in contact with him and has gone to Israel a couple of times. My mom has stated that she goes to Israel because she "loves" it there, but I have a feeling that it's not so innocent. My mom has expressed to both my sibling's and I that this guy is/was the "love of her life" and she can't help it but it's hard for her to "not talk to him"... Apparently. But this entire time my mom has expressed this to me, I've told her that I don't feel bad for her. That she knew this guy was in a relationship and is continuously in contact with him. And that she paints herself the victim here. Now, my mom is back in Israel for a couple of weeks for her 53rd birthday. She told my siblings and I that she had "lunch with her friend" (tour guide) and his S/O found out and to just block her from everything. Apparently this woman found some of my families' facebooks and has been "sharing" their pictures and writing stuff about my mom. Again, my mom is painting herself like she is so innocent while my older sister (25F) is agreeing with my mom and feels so bad for her. My younger brother (20M) and I do not feel the same way. We both feel that my mom is not innocent and why should we treat her as such? She got herself into this position and now she is dragging her family into it. Both my mom and sister feel that I am being unfairly harsh and think I'm being "cold-hearted"... But I guess ever since this started back in August 2014, I've held a small grudge against my mom because I don't go to her apartment as often as I should (probably once a week) Am I being unfairly cold about this whole situation?