
Yeah. He is Lorel's (the 2nd oldest brother of Gallahan)son.
Laranne is his oldest daughter.
Bellesac is the son of Vieze and is a carbon copy of his shitty father. And the reason the other cousin seems to be a shitty person is partly due to Bellesac's influence who is older than him. And asserts himself as the "oldest son" among the cousins.

Shananet - Only daughter. She is the mother of the blond twins
The twins take her surname.
Vieze - Oldest son, a shitty person. as stupid and arrogant as his son - Bellesac.
He is married to the cousin of the Empress who is from an old noble family and from what Tia describes, she is a nasty snob just like her son.
Lorels - 2nd son father of Laranne and Astalieu
Gallahan - youngest son and father of Tia.
She isn't the sister of Bellesac but of the other shitty cousin.