My ex's mother friend requested me on facebook?

Him and I broke up about 4 years ago now because he cheated on me and I am in a new relationship that's going on 2 years. I didn't know what to do at first when I got the request so my sister told me to deny her because I shouldnt be involved with something that was in the past that didn't work out. Before my newer boyfriend and I met, my ex and I made amens but I would never go back to him. I forgived but didnt forget. Im feeling kinda bad for denying her but I know that my guy now wouldnt probably like it (and I probably wouldnt either if it were switched around). She was a sweet person and I wouldnt want her to think I hold a grudge against her. Should I just leave it alone or write her a message to give her a better understanding?