I'll do some more, but only the people who I have a decent inkling of.
Yoshi - The brother who seems very sensible on the surface, and very into his studies, but once you get a bit of beer in him he turns into that guy who embarrasses everyone else with crazy robot dancing.
Agent Griff - The Pizza guy who interrupts family affairs at inappropriate moments, and rarely gets tips. Except at Christmas, when everybody is in a giving mood, and he makes a small fortune. The next day he's likely to blow it all on a new PIZZA SCOOTER!
Naridill - Obviously the snarky, pretentious foreign kid renting the basement. Only not so much "renting" as "locked inside". On Fridays, as a special treat, he gets a bucket of fish-heads. It's not exactly clear why he's locked in the basement, but whenever Worbah goes down there, you can hear the sound of quiet sobbing.
Worbah - The sock-stealing bastard son of an unholy union between a crack whore and a dog. His kleptomaniac tendencies land him in trouble with others.
ilex - Worbah's accomplice in crime, and usually the first one to start singing German nursery rhymes whilst claiming that they're actually death-metal.
Cleaning Agent - The kid who always finds something to argue about. If you told him that the sky was blue, he'd argue about which shade. Possibly related to Gollum.
Seff - The brother who sits in a corner wearing a trench coat, and, oddly, never has his hands in plain sight.
Alex - The distant cousin who shows up from England every now and again to ask you a bunch of annoying questions, then samples all the food in your house, uses the last of your toilet roll, and disappears back to the mother-country before you have time to realise what's happened.
okiN - The cousin who pretends to be sane but secretly harbours red polka-dot slipper-wearing tendencies.
Velmu - The drunken, absent father who never sees his kids, pays the maintenance or sends birthday cards.
Leprechaun - The good-looking brother who everybody envies, and they secretly covet all the girls in his class.
Bulle - The sister who does what she wants and doesn't care what other think, and harbours a not-so-secret longing for Lost-Lamb.
Lhorkan - The quiet cousin who makes wise-cracks when you were least expecting it.
Raz - Token black guy and sexiest person in the family.
Cymro - The cousin who lives in Wales but doesn't live near sheep, so instead he keeps a little fleecy sheep plushie in the inside pocket of his coat. When questioned about the mysterious bulge sticking out from his pocket, he just says "it's porn". Usually it works, and no further questions are asked.
Nairagorn - Deadbeat grandpa with hair that has a life of its own, and is possibly responsible for the disappearance of missing children over the centuries.
Llew2 - The fairly distant cousin who disagrees with most everyone else, but reserves telling so them for special occasions. Strangely, nobody knows what happened to Llew1, but it's possible Llew2 buried his body in a shallow grave and then took on his identity.
Redcoat - Mic - The student brother who's always boozed up. Possible he was born that way.
Morbo - The lost puppy who wandered in one night, dodged a few kicks from Bugman, settled under the kitchen table, and never left.
Ingolifs - The visiting professor who covets Cymro's sheep plushie.
Hayabusa - The skinny little cousin who needs to bulk up.
Freebird - The distant cousin who stands outside your house in the bushes, completely stark naked, making little children (and women) cry.
Gculk - The cousin who frequently gets used as a pinata.
