A really tempting and easy thing to do is make your character related to someone who's already there. This is fine, but it can really cliché your book.

I see a lot of Marvel fanfics with Tony's daughter, or Teen Wolf fics with Scott's sister. It's a common thing and can take away originality, especially if all of them seem the same.

If it's crucial to your storyline then do it, but give your character their own story, you're making it for the character afterall.

Consider making your character be related to a minor character, like I did with my Shadows series.

Really quick I want to touch on Steve Rogers. If your character is his sibling, how? Was she frozen with him? Did HYDRA get them? DO they have powers? Did they experiment on themself? 

Don't just make your character related to someone. They should be more than that. The story should truly be for them and make the reader feel like that character could be in the fandom for real.

Hope you enjoyed! Feel free to request and ask questions! Love you all! Byeee!

~ Lydia