I'll relate a personal experience that may apply in some way to your situation.
My dad passed away years ago. I was very close with him. I spent a lot more time with my mom after his death trying to help her out, be good company, support her. My mom, a wonderful woman, met someone a couple of years after my dad passed. I reacted very poorly to her new "boyfriend". Even at my mature age. I had complex emotions about it that I'm not even sure I can verbalize. I felt she betrayed my dad. I somehow felt betrayed by her. There was something just not right to me about her merrily going on with someone else. Most of all, I think I probably felt tossed aside since she was now spending more time with him. That's the best I can explain it.
I was stubborn in not wanting to see her when she was with this man, I didn't want to have anything to do with him. I acted like a two year old for a few months. How I cringe when I think of how stupidly and immaturely I acted. Then I came around and got to know him. What a wonderful human being! I even told him a couple of years later that he was like a second father to me. That made him feel wonderful as he always wanted a daughter. He has since passed on too but I know if my mom were to meet another man, I'd be all for it.
Your sister is still grieving. The dynamics have changed between the two of you now that you have met someone else. She has depended on you like she really can't with any of her friends or acquaintances. In my (maybe uneducated) opinion, it's probably not easy for her to accept this changing situation and relationship because of her grief. It has been the one dependable and comforting aspect of her life.
Like others have said, I would lay low with your new bf and continue to see him on your own time. I have every confidence that your sister will come around...in time.