My sister told me she had an affair and I don't know how to take it

I’m having a hard time I got a bomb dropped on me yesterday. My sister told me she had an affair on her almost perfect husband. I mean he worships her and has always been there for her their child and our family. No matter what. The reason being communication and growing apart. Yet she never told him this. She tried counseling while still in contact with the guy she was seeing. She says she just misses the guy so much but never left my bro in-law just wasted his time, with him thinking counseling was helping them when she didn’t feel it was. He is willing to forgive and make it work and work on the issues he knows about now. But she says it’s suffocating and weird. Tht the connection isn’t the same with this new guy, isn’t aways all love and perfect in the first stages. I don’t know how to support her when I think she lost her mind. If my hubby did 1/4 of what this guy does and is willing to do my marriage would be :100: different and better. I don’t think she realizes what she is losing and the impact on her child and what if it blows up in her face and she regrets it. This guy was supposedly cheated on but yet he did it to her husband what devastated him. She says he respects her but how is that when he crossed a line with a married woman even if she wanted it. Ugh I’m all over the place with emotions. Not to add it sounds like instead of figuring herself out she wants to move in with this guy right away.