I think it's a harem dynamics thing - Athagan calls MC son, his wife never does. Lia tells us that Solange is happy and trusts us with her daughter when she can't journey to the capital herself so there's at least that.
The thing with Athagan and Liliana is different. Athagan adopted MC as an adult for political inheritance reasons.
Not saying Athagan is a bad father or anything he's not, he considers Maria his granddaughter and treats her as such.
But Liliana has no reason to adopt MC nor refer to him as her son. Not before the marriage to her daughter and not after. And none of that is getting into the meta knowledge that Liliana will be joining our harem at some later point, meaning she can't have been calling us son and us calling her mother etc.
So unless there are plans for Solange to marry a son of ours we have in the future, which would be weird, there is no reason for her to not be actually treated like our other children and them all be referred to correctly.
Solange's mother, Lia, is one of our women, one of our wives/concubines, and will be giving us more children. It would be cruelty bordering on barbarism to have Solange's younger sibling(s) by her mother be her only sibling(s), while all the other children, including Solange's younger sibling(s), get to to all be siblings to each other. Like if that ever happens, I wouldn't blame Solange for turning evil.