About goldfish siblings?

Question

do they mate their siblings

and i have two sibling goldfishes and one is alot bigger then the other goldfish and they dont fight so its okay.

but why is the other goldfish alot bigger then its sibling?

Answer 1

Goldfish do not differentiate between siblings and none siblings, so any male will spawn with any female. Under certain/special controlled circumstances this type of breeding is done by specialist breeders on purpose to establish a "line" or type of fish.