Is this normal?
Highly unlikely, I guess you were just unlucky.
This would be grounds for taking a second wife though, perhaps that could be implemented so we can still have an heir even if the first wife doesn't provide any.
Actually, I just discovered I didn't have girl twins but it seems 1 girl 1 boy. The confusion was the boy looks like a girl.
Would be nice to have the info about gender in the character details.