It isn't going to be incest!
My guess,based on many years of watching TV dramas,is that one or possibly both of Leo's annoying children are going to turn out to be NOT Leos.
The boy has two blond parents,fairly unlikely that he'd be so dark and the girl is most likely the child of the random stranger that just turned up,hence the line about Leo being forced to take the girl on the day out.
Sir Guy Grand-I like school of Rembrandt
Youngman Grand-St.Rembrandt's high