Dealing with Marital stress with help of ex boyfriend can only complicate things, esp in case as your sister's. Better see a psychologist or a good martial counsellor ( assuming stress is mainly marital) , with whom your sister can confidently share information and they will be able provide non biased opinion. Whereas a ex boyfriend usually is biased as he is emotionally involved with PT and so may hinder rational decision needed at this time

I see this as sheer insecurity and lack of clarity. She is confused about herself. I suggest her to visit a psychologist who is also a Marriage Counselor personally.