is saying "in a sibling of three" makes sense
like i want to use "in a" the same way you'd use it in "in a family of three"
like what i'm trying to say is "in a sibling of three the middle child is usually the first one to break up fights."
if i got it wrong could you guys please correct my sentence for me
You'd have to say something like "In a family of three children, the middle child ..."