What is the point of Izek and Ireena being bio siblings?

I'm just at the stage where the party is about to take Ireena and Ismark to Vallaki. In the upcoming session I have the opportunity to have Donavich tell them about Ireena being adopted, but the only reason for that plotline being there is so when they meet Izek in Vallaki you can reveal that they were siblings. Sure, you get some creepy dolls, but otherwise it all seems a little pointless, and detached from any other plot. Have any of you guys done something more significant with this? I'm considering just having Ireena and Ismark to be natural siblings.