EXAMPLE is good. Prayer and mind your own business are good too. Pray for her that she will clean up her act. Pray that her husband -family, will not be hurt by her cheating ways. Pray for yourself that you will be a good example and that you will fight the depression and the urge to live her life for her. You can tell her how you feel and give her reasons to take care of her family life. That's about it. Unless you want to be the barer of bad news, if so, tell your daughter that you give her a certain amount of time to stop the affairs and get help or ask for a divorce or tell her husband before you do. Then keep praying until something happens. (PUSH) PRAY UNTIL SOMETHING HAPPENS
I know it hurts you but those are the choices you have. I hope the pray works for you.