"У вас немає брата."

Translation: You do not have a brother.

[Caterpillarman]

actually I do

[Maximos_Ens]

Me too.:)

[Caterpillarman]

haha

[UKTerry]

Why брата not брат. It would help if you explained why I am wrong

[Maximos_Ens]

Because it's genitive here. When you see Увас/нас/мене/тебе etc it's always genitive

[SpeakMeSE]

(Someone correct me if I'm wrong here...)
It is correct that the preposition "у" (or "в") indicates possession which is why we say "вас" (genitive of "ви"). "Брата", on the other hand, is in genitive due to "немає", i.e. due to the absence / lack of a brother.
"У вас є брат." but "У вас немає брата."

[Ukurainajin]

‘У/в’ points at some place in general. It usually requires Accusative for direction (into, to) or Locative for location (in, inside). But when used with Genitive — you are right about it — it can also mean both possession and place (at, by). Actually it's kind of a mix (a ‘place’ that has something at it or with it) and you translate it into English depending on what makes more sense:

• Possession: У моєї мами є кіт (same as Моя мама має кота) — My mom has a cat. У цього дерева три гілки (same as Це дерево має три гілки) — This tree has three branches.

• Place: У моєї мами живе кіт — A cat lives at my mom's. У цього дерева на гілці птахи (same as На гілці цього дерева птахи) — There are birds on this tree's branch.

[spicy_wolf]

Sounds sad :(

[Caterpillarman]

A wolf being spicy sounds even more sad haha