Charlie-Cecilia
This is based on the Happiest Season movie, but you don't have to have watched it to understand this. Though I highly recommend the movie :)
Alec has a bad past with Christmas as his parents died on Christmas three years ago. Alec now looks after his siblings Izzy, Jace and Max. Alec has been in a relationship with Magnus for five years. But Magnus is terrified to tell his parents that he is Bisexual. He wants Alec to spend Christmas with him and his parents. Magnus also invites Alec's siblings to come so they don't have to be without Alec. Alec thinks Magnus has now come out to his parents, but he doesn't know that was a lie.
However as soon as they arrive at the house Magnus tells Alec he lied about coming out and asks him to keep their relationship. Though Alec is hurt Magnus lied, he reluctantly agrees to hide their relationship from Magnus' family.
Magnus' mother is alive in this AU and I have created a sister for Magnus.