Can you please tell me what to do about my ex-brother-in-law?

My sister went through an ugly divorce last year. Her husband was cheating on her with his co-worker. I have been divorced for the last 15 years and have never remarried. Last week my ex-brother-in-law called me and asked if I wanted to go out on a date with him. We weren’t even good friends while he was married to my sister. I cussed him out for all I was worth and hung up on him. My delima is, do I tell my sister about it or not. I’m afraid if I do tell her it will start a huge fight between them, and if I don’t tell her I’d feel like I was lying by omission. They have 2 daughters together so they still have to have contact. Can you please tell me what I should do?