I'll try.
Ok, so first you have to click to see your prophile (click on your name, which is on the left or at the top of the page).
Then you'll see some of your friends shown on the left of the page. If you pass the mouse over them, you'll see that it appears this small pencil on the right of where it's written "Friends (number)".
Then just click on the pencil and you'll se a page where you can edit your friends and add family members. I don't know if it's the same there, but for me there's this sequence:
- Relationship status
- Add a family member
If you see this "Add a family member", just click on it and then it's just adding and choosing if it's a cousin, brother, etc.
I don't know if I described it clear enough. 
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Your Aspie score: 161 of 200
Your neurotypical (non-autistic) score: 45 of 200
You are very likely an Aspie
"A man is as free as he chooses to make himself, never an atom freer"