Wow! This story has a lot of great emotions going for it. I would totally watch it on Hallmark!
Just to clarify. You said: Her friend died and the wish is part of a request to the group of friends that was sent after she died.
Is the niece’s mom the friend who died? Or is it someone else? Could the wish be something that the mom wanted for her daughter? Instead of really digging into the main character’s GMC, look at the GMC of the person who made the request. What did they want? Maybe the mom stole a family heirloom in order to help her fund her original trip to NY. She always felt bad about it and revealed to her group of friends what she did with it. Your heroine didn’t pay too much attention to it until she got to know the brother and the niece and realized how important it was to him. It doesn’t have to be something valuable…more sentimental really (but maybe sister thought it was valuable). The heroine and her group of friends could help find it and return it. And in doing so, maybe restore our hero’s faith in Christmas. To get it back, the heroine may have to break some dates with the niece (making the hero think she’s flitty and unreliable like his sister was) or something like that, which could really up the conflicts. In the end, it could be returned anonymously but of course he will find out. 🙂
Just some thoughts. 🙂 I’ll keep thinking.