To not want to go to my brothers party?

A waste of petrol to drive an hour and back to spend time with your Brother on his 30th Birthday. Really? Do you not like him much? I don’t think that’s much at all. Sounds like you are just making excuses not to go.

OP - it sounds like you’re in a very bad place mentally at the moment, and that’s the reason you don’t want to go. Your family will see through any of the excuses you’ve posted here - they’re all extremely obvious. Just be honest about your mental state with your family. It sounds like you need their support.

Ah, so it's not really just about the cats then, it's anxiety around other things related to the event. Well, I think it would probably be better to be honest with your brother and just say that it's too much for you/overwhelming/whatever because of your anxiety. Making it about leaving the cats sounds a bit daft, whereas they "should" be more understanding if it's because of somethung they know you suffer with.
EDITED to add that I have suffered badly with anxiety around social events for years, so I do understand what it's like, but I still think it's better to be upfront about what's really stopping you, than make implausible excuses. I've done that myself in the past and realised it doesn't work.

It’s not a waste of petrol if it’s going to an important family occasion though? To see your brother on his birthday and join in a family event?

I would stop with bogus excuses (not being able to leave cats, a waste of petrol) and be honest about what’s going on.

The cats are a pathetic reason. Social anxiety is a valid reason, but it’s worth pushing yourself. You will have family there and it will be important for your brother. I’d drive then you can leave when you want. It’s a bit mean to say your brothers birthday is a waste of petrol.

Head over there about 4pm on the day of the party, and then leave at 6am the next morning to drive home. We’re only 6 weeks off from the longest day of the year - you won’t be driving in darkness at all.

And ‘a waste of petrol’ to drive 1 hour each way to see your family and celebrate your brother? That’s your worst excuse yet, and pretty insulting to your family, so I suggest you don’t bring that one up.

OP, it sounds like you just don’t want to go to the party and are looking for an excuse to get out of it - and that’s what your parents/brother will hear too.

An hour’s travel is really no big deal. You could feed your cats slightly early that afternoon and travel to your parents, and then leave by 6am the next morning and feed them their breakfast at 7am. As long as they have food and water, cats are absolutely fine being left by themselves - people get cats because they’re not home enough to have a dog.

It seems like you have anxiety about social situations and don’t want to go for that reason. So just be honest with your family.

It's not far. I'd drive both ways and stay a few hours.

But… they are cats.

Well, I voted YANBU because you can't help how you feel, but at the same time I think your anxiety over leaving cats for one night is a bit OTT. I have two much loved cats and I have one of those automatic feeders for use when I have to be away for a night or two. You put enough food in for the time you'll be away, set the timers and the feeder just uncovers the food at the time you set. They're brilliant, what about something like that?

Hi OP could you drive there not drink and then drive back so they aren't alone overnight? Alternatively could you book them into a cattery for the night? I think if the relationship with your family is generally good then you should try to go, I'm a animal lover too but missing a milestone birthday because of your cats does seem a little unreasonable. Is there perhaps an underlying reason why you don't want to go and the cats are just an excuse.

Why can’t you drive over, attend the party for a few hours then drive back the same night? Am I missing something?