how would one say "i have"?

there is no direct way of saying "i have" (hindi has no verb "to have"). there are two ways to express possession:
1. in this situation " Ek bhai aur ek behen" you would say:
meraa ek bhai aur ek behen hai. OR mere do bhai hain (i have two brothers) OR merii do behenein hain (i have two sisters). of course in a conversational situation, your answer would be perfecly respectable (as it would be in english - "how many brothers and sisters do you have?" - answer "one brother and one sister")

2. where you are talking about possession of objects, you would say for example;
mere paas ek car hai - i have one car (there is no point in analyzing this sentence as it will only tend to confuse - just learn it as a paradigm)