overprotective cousin?

You ignore them, block all of them and leave the post.

It's special and dedicated to your younger cousin ; it obviously means a lot to you and him. If they had a heart, they wouldn't badger you about the post. And they don't have any rights to the photo, as it's not on their phone. Just because she's his "sister", does not mean anything. The meaning is the same as a piece of newspaper on the floor - zero. Therefor, they really look silly at the end of the day.

As annoying it may seem, keep on blocking them. Eventually, they'll give up. They can't keep harassing you forever......The next step is to talk to them about the situation, once they've settled down..... Of course, this requires unblocking them. Tell them how much it means to you AND him. And how special that day was to you guys. Also say that she should respect that and she'd likely do the same if you had a disabled sibling.

If they still don't see where your coming from, drop communication about the subject for awhile. You know you posted the photo because you love your cousin, wanted to showcase how amazing he looked on that day, how proud you are of him, and how much you love him.... If they are going to act selfish about it, then they can f off as there is nothing wrong about this.

Overall, I'm sure your cousin who had the post about him, appreciates the post and likely feels honored. He also does not want this negative attention about a photo. And if you come to think of it, your probably not the only one who posted a photo of him on social media. I'm sure other friends and family members have on facebook (or other social medias).

Goodluck, and don't let them get to you.

i dont think youre doing anything wrong, i would leave it up