That wasn’t his sister that was a descendent of his mom’s tribe remember the beast live longer than humans do he was born some centuries ago.
okay this is going to be a long ride
so from what I can remember in my head right now in the spot I'm not even trying to look through the manga to answer this question
here we go and there is a part of the manga where the counselors that serve Leo the king said I quote "the previous King (Leo's dad) had made peace arbitrarily with the human more than 200 years ago" but get this in the anime when princess Amite from the reptilian clan went to ask Leo's hand and marriage that 200 years ago exactly was Ozmargo and her country were at war but then Leo turned her country into a vasel state of Ozmargo and when she said that a shot clip was played of the king (Leo) and look to be 15 but when the counsellor said more than 200 years from then till now Amite said the same thing from them till now but Amite sad exactly 200 years the counselors said more than 200 years it could have been closed to 300 which means after the king died more than 200 years ago Leo took over, grew up a bit and then took over Amite's country and now we're in the present time line.
so we can say Leo is between 300-200 but let's just say 280, so unless Anastasia can live for 280 years from when Leo was born then yeah she is his sister let's also talk about Oslote Anastasia's friend and they knew each other since childhood so THAT MEANS OSLOTE IS LEO'S BROTHER BECAUSE HE SHOULD ALSO BE AROUND THERE AEG.... RIGHT.. RIGHT and plus don't you remember the villagers thought Anastasia was a which if she's that old don't you think they would have already burnt her to dead
so my final answer is no she is not Leo's sister
PS: and now think Leo is 280 years old and Saraphy is 15 years old a 265 Year gapb Leo is gangster for that
Wait so did he never figure out he has a sister?!