The word you want is "siblings". How many siblings have you got? You probably meant to say "encompass" or "include" when you said "englobe". ;)

How many children do your parents have? / Are you the only child in your family?

Siblings is the word, but it's still probably more common to say, "Do you have any brothers or sisters?" or "How many brothers and sisters do you have?"

How many siblings do you have?