Sounds like the problem may be hereditary. Similar to my MIL and W. My MIL's H calls her Flash and my W, Flash Jr. He says that he got the original and I got the carbon copy.
Unfortunately, your W may have unknowingly learned a lifestyle from her mother that she's now repeating in her adult life. It may feel normal to her. I've since learned that my W has never let a man go until she had another one waiting and ready to take her. But that's part of the BPD in her.
tough call, I gave my W many chances. I forgave her several times; but these last 6-8months have been it for me. Unless a miracle from God saves her and this M; I'm done. I am enjoying DB'ing though because it's been healthy for me to find myself again. It's helped me let go of the woman that I thought I loved and thought that I couldn't live without. I found out that I do survive without her and that other women still find me attractive. There is life after M to an adulterous wife and crappy mother.
Don't take my sitch for advice on what to do with your sitch. She may not be a serial cheater; but you have to be wary with your MIL's history. Keep Db'ing and watch for signs. It may be a while. Don't rush back in if she comes back; have boundaries in place.
Keep working to get happy without her; it's your only hope to survive this mentally and emotionally intact.
