Sister sister 😒

My little sister recently moved in with me she's going through a divorce. She has been separated for more than a year now. She started dating a couple of months ago. The guy she was seeing did the same thing to her as her soon to be ex-husband..cheated on her. So she just told me she's dating someone new. Don't get me wrong I want her to be happy but at the same time I think she doesn't need anyone right now. She needs to her feelings together and do what's best for her and my niece. I love her to pieces but I get frustrated that she worries about a new boyfriend instead of her future. Maybe I'm wrong idk. I guess I'm asking for a way to talk to her and tell her how I feel without her getting upset? Thank you lady's !!

Update: Thank you for your answers I'll reply here. I should have been more clear I don't mind her dating it's her life she's old enough what bothers me is that she takes my niece when she goes out with her dates after she said that she wouldn't just in case it wouldn't work out. What if my niece gets attached to this guy too soon and at the end it doesn't work out? Maybe you guys think I'm being negative about this but every time she gets hurt it hurts me more because I feel like I didn't protect her.