brock and kairel they were raised together as small kids until kairel was abandoned by her mother, because she looked too much like a normal human. and she was sent to live on earth in a whole other era with other's like her but when she turned 18 she encountered a werewolf and was bitten causing her to finally became weird creature like her birth mother wanted.but being that pearl earth is in a universe of its own when she finally did return she was only a preteen instead of an adult.

Brock was not raised on pearl earth either, his mother took him to earth in the hidden city of waterfall north
and was raised by a single mom until she met and married her S.O and had his little sister kamira, brock did not like all these changes and was offered to live
On pearl earth with his father and aunt who adopted him as her own. Both kids were more like their aunts rather than their moms

as to why brock and kairel are called cusling's is ... well same dad different mom's who are indeed sister's