in a

[purpletouch]

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

[Raymott]

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

No, it's not right. A sibling is a brother or sister. You can be in a family (part of a family), but you can't be in a sibling. There are words siblinghood and siblingship but they refer to the relation between one sibling and another, not to the group. I can't think of a word for this concept in English.
You'd have to say something like "In a family of three children, the middle child ..."