As a person who has done a ton of fanfic related stuff and been in a lot of fanfic and roleplaying games, forums, groups that sometimes yes dealt with a preset universe like MLP:FIS is I will say nothing is ever bad but when you do something it has to be done well.
In the world lore alicorns are rare and their creation should be equally rare. If you want all that power you have to work for it, and work hard. If you can make an OC where the fact that they are an alicorn isn't the first couple of things people say when they describe your character, if there are definitive traits and quirks and it feels like a real person then great all the power to you.
If you or anyone else ever say the words "I'm an alicorn" in the opening definition of the OC you need to start over.
And of course a ton of people want to make Alicorns, OCs are an escape and alot of people don't want to dream small, they don't want to be not special when they go there. It could be because fo self esteem, or real life problems, or just the fact that they are a middle child and feel alone. Sometimes...sometimes people just want to be the biggest most awesome thing in the room because they want to feel good.
As for being related to the mane six...I personally recommend you have two versions of your OC if you want that. One where you are, which is for you and you alone where you can enjoy and share the bond you want to and a version of your OC you represent to other people in roleplays. Alicorn or not. Being the family of main characters is an insult to a lot of fans because it can appear self-indulgent, small sighted or just shatters their understanding of dynamics.
Just my two cents...