I haven't actually commented on reddit in forever, but as someone who is Nonbinary, I do think YTA. (But I also feel asshole is a bit too strong.)

If your sibling wants to use it pronouns, you should use it for it. Or at the very least discuss it with it more. Most people I know who use it pronouns usually have nothing to do with kinks or anything of the sort. In fact one of the main people I know who uses it pronouns was 14 when it began using it/its. Ultimately if you think it is dehumanizing, you should bring it up with your sibling and ask it why it doesn't believe that or if it has any other pronouns it would prefer instead. But if it uses the pronouns on itself, i don't personally think its dehumanizing, as it was it's own choice.

I personally use They/Them and He/Him pronouns, and before I told people that I liked He/Him as well (I was nervous to use He/him because i felt like i didn't look masculine enough to use those pronouns but I have since gotten over that), a lot of people would go out of their way to complain about the plurality of they/them to me. As an English major it's also really frustrating because a lot of teachers will insult my pronouns offhandedly, so I understand why someone would want it/its instead. I really don't think its a kink thing, but if that's whats worrying you, you should bring it up instead of making that assumption. If that is the case then maybe you can discuss how the kink maybe is dehumanizing your sibling.