This one was suggested by SuperDCat:

Ever get the feeling that you have a sibling or child that you never knew you had? No? Well for cannon characters all over, this is the truth apparently. They have siblings that for some reason they had no idea they were related to.

I get if you write a fanfic with an OC sibling/child, but have the main character actually know that they are related to that OC. The "unknown relative" have been crazily overused, especially when there is no way that the MC wouldn't know that they had a relative.

Anything thing about relatives, they can't just be carbon copies of the MC. Everyone has their own feelings and looks (even if they are identical twins, they don't inherit all of the same personalities). Make your OC different and their own person.

Announcement:
Thank you so much on 100 likes!
(Now I won't waste any more of your time)

Peace out,
Ellie