What should I do?

Recently my sister told me something about her love life. For context my sister is seventeen and has been in multiple relationships before this, our parents are also divorced and have since remarried that's why we don't see each other as often as we used to. So, up until now my sister and I haven't really talked much since she lives with our mom while I live with our dad, today she went over to spend the weekend with us and get some stuff for her trip the same month. We spent some bonding time with each other and went out driving for food/snacks. She started to open up to me about her past relationships which lead her to tell me about her current boyfriend, at that moment she revealed that the guy she was seeing was actually 26 all the while she is 17. She then went on and on about how great the guy was and how much she loved him, apparently she even introduced him to our mom and she was OKAY WITH IT! I have this nervous feeling that I should warn her about dating guys in that age range but everyone else seems to be okay with it so i don't know what to do. I don't want to overstep since my sister is finally opening up to me but I also don't want her to do anything she may regret.