Who is to say that Laas wouldn't have stayed at the Link? Since he too despised solids, it was more of a better fit than Odo who obviously welcomed them. this was essentially the reason why he rejected the Founders, not really because he was good and the Founders were evil, but he loved Kira and wanted to be with her. I'd imagine had Kira died prior, then he'd have little else to live for amongst solids and go back to the Link. Also, Laas only knew of the Link after he met Odo. Up until then, he had not means to know other Changelings even existed. One cannot catergorically state that as Laas was greeted at the Link and told "Welcome home" he'd respond "You're my people? Nah, not interested". It's likely he may share his people's hatred of the solids, and do what he could to subdue/exterminate them. Remember, he killed a Klingon and showed no remorse since he was a solid.
It's likely then as he was told "Welcome home" he would respond "It's good to be back after all these years. Let's go kill solids together! "
It's that as the Founders were Laas' people too, why should he be denied the opportunity to meet them? It was his home and origin, and if he chose to go, fine. They let Odo go, and all Founders seemingly have equal standing/choice (I see no evidence there are Founder leaders). Odo essentially was a hypocrite, since he denied one of his own species the opportunity to do something he wanted all of his life.
Just seems that if Odo lived his life in the pursuit of justice, then he should have at least shown Laas what the Great Link was. Why deny somebody their own origin? As for DNA ties, well I'd say a priori most species do have such a thing. Of course this is not absolute, but despite PC people disagreeing, most species do share utmost allegiance with conspecifics.