Most people, especially when they have a new significant other tend to prioritize that person over siblings. Sister will eventually normalize when her romance enters a cooler phase and she will remember her sibling. I assume you are young. Take this as a life lesson - Siblings and friends will sometimes find an interest that occupies their life for a time. Usually this passes to a degree but once they form their own family, then their own children and spouse or significant other will be at the center of their world. The probability is it will happen to you some day so learn from this - Yes, a romantic partner and your own nuclear family will eventually be at the center of your world but that does not mean you should completely ignore family and friends. But this is something people can seem to do - It’s just part of growing up. In the long run people need both family and their love interest. Sometimes one takes precedence over the other for a while. But unless it’s an unhealthy family, both will ultimately be important.

Maybe the new boyfriend is intercepting the calls? Maybe he changed your name in her contacts to someone she doesn’t want to talk to?

Screenshot this and sent it to her.