Sewer monster

Thumbtacks on the toilet seat.

You're welcome.

hide his fatty food then

So get up and move. There. Problem solved.

This post is just about his bathroom habits, right? Why even bring his weight into this, it's not relevant to the story at all. Doing that just makes you look bad, not him.
Just for that, I'd say this is more of an F his life bc it sounds like he's got a rude sibling who says shit just so strangers on the internet will insult him 🤷🏼‍♀️

Clearly there is problems between the two of them and situations we don’t know.

I think most people with siblings have said some pretty horrible things about each other when annoyed. most tend to be forgotten or forgiven down the road. siblings fight, then they get over things. It sound like OP and their sibling have issues that cause tension... I doubt it is one sided, but eventually it should pitter out even if it is after on moves out.

Humble brag with your multiple toilets.

Saran-wrap over the toilet bowl. Won't help the smell, but it'll give him a surprise.