Perfectly understandable to want to have your own family thing with your LO.
Maybe work out what is important to you about that morning eg opening presents, going to church, having family breakfast etc etc Then invite your sister to join you after you've done your thing (if you want to)
A first Christmas is a big deal. I remmeber with DD1s first Christmas Eve I sat in the lounge with the Christmas lights on, some really pretty Christmas music and nursed her before bed. DH was yabbering about something from the next room, and I just tuned him out and snuggled my girl, but when he came to see why I wasn't answering him he absolutely got it.
So I guess I'm saying, those moments are precious, but if you focus on working out what you want and creating it then you should still have space to reach out to your sister.