Is "tumhare paas" incorrect here?
पास (pas) can be used only for objects/animals. Not for people as you do not "own" them.
You cannot abbreviate "you have' in this context to 'you've' as is shown is the correct answer
Why not this disposition?
तुम्हारे भाई-बहन "कितने" हैं?
Why not "tumko kitne bhai-behen hain"?
why no पास ?
How many siblings are you
Because that sentence does not make sense ¯_(ツ)_/¯
Why thumare here? Why not thumko or thume?
In British English we'd probably say 'How many....have you got?'