New Person; With sick cat(s)

Ok, I am new and found this site while searching for things to help my cat(s). I have one 1 year old s peiamese, he is known for having hairballs...CONSTANTLY...Like once a day, but anyways today I walked up the stairs, and he ran from me (feral cat) and he started to have a hairball. I put the paper on where he was, and he ran from that. Then he puked, he usually pukes liquid and vet says this was normal. However, he puked green-yellow liquid on the stairs. I cleaned it up, and smelt like a rotten egg you just cracked open. There was also some fur in it, so I figured it was a hairball. But he went upstairs, and just layed there in the middle of the floor. He didn't move when I tried to pet him, so I am just worried he is sick. Also, if he is, his "brother" who is two years is constantly licking is face and rear, along with my siamese who does it all the time. The two of them have been acting wierd all day. I am gonna check the litter tomorrow, since I cleaned it twice today. Should I change the litter? I just need ideas.
Also, could this be from milk that was on the table he may have drank?

When a cat has a hairball, they puke up hair, not clear liquid. I would get your kitty to a new vet.

Is the 'brother' licking his face and rear so constantly that it's causing fur loss? If not, I don't think I would worry too much about it, as cat are just very fussy about being clean. It's normal for a cat to spend hours cleaning itself.

There are too many factors to really say what it wrong, so the best advice I can give is to get him to a vet.

Also, welcome to the forums