R
I don't think there's anything wrong with telling your mom. However, if your sister is 25, you need to partially accept her decisions. That doesn't mean you need to approve or condone them. And you don't need to stop mentioning now and again the dangers of online relationships. But ultimately, she's an adult and needs to make her own choices. Don't chastise her every time the two of you discuss the relationship. That will only push her towards him and stop her from confiding in you. But you don't need to pretend to approve either. Try to keep a neutral attitude when she discusses him, so that she doesn't draw away from you and towards him. However, if you hear that he's saying or doing things that are abusive, there's nothing wrong with pointing it out to he.
I know it's hard. My sister was in a psychologically abusive relationship during high school. Everyone could see it but my sister. Her situation was different because she wasn't an adult. But it was incredibly hard to watch someone who I knew was intelligent act in ways that defied logic. However, what I found was that the more I tried to talk sense into her, the more she seemed to turn deaf to what I was saying. If I could do it again, I would have taken a different tactic. Thankfully, they broke up and, while she still does things in her relationships that I don't approve of for Biblical reasons, her current boyfriend is a decent guy. I can't be sure that will happen with your sister. But I do believe that the best way to go about it is to allow her to make her own choices. Let her know your views but don't shove them down her throat. If she can see you as someone she can talk to, she'll take advice from you and maybe some of your views on her relationship will sink in.
I know it's hard. My sister was in a psychologically abusive relationship during high school. Everyone could see it but my sister. Her situation was different because she wasn't an adult. But it was incredibly hard to watch someone who I knew was intelligent act in ways that defied logic. However, what I found was that the more I tried to talk sense into her, the more she seemed to turn deaf to what I was saying. If I could do it again, I would have taken a different tactic. Thankfully, they broke up and, while she still does things in her relationships that I don't approve of for Biblical reasons, her current boyfriend is a decent guy. I can't be sure that will happen with your sister. But I do believe that the best way to go about it is to allow her to make her own choices. Let her know your views but don't shove them down her throat. If she can see you as someone she can talk to, she'll take advice from you and maybe some of your views on her relationship will sink in.