nope, even if he was their step-father (he wasn't) that ended when he divorced from ex-wife.
Actually, Frank was Becca and Lilly's stepfather, but yes, that ended with the divorce. But what falls under that is the relationship that Lilly and Abby had with each other when Shannon and Frank were still married. Both were step-siblings and both had a scene together in Lilith's Longing, which took place (the scene, not LL) at a time when Frank was not yet divorced from his ex-dragon.
By the way, I'm currently going through the storyline again and Becca once said, when she explained the pecking order thing, that Lilly crossed a line once, so Becca felt forced to use the one thing that could massively hurt her at the time. Are you also wondering what that was? My theories on that:
Before Frank and Shannon got married, little Lilly was a mess. I can't imagine that at the time she would have been able to push Becca, who was rather introverted at the time, to the point where that would have been necessary, even though she mentioned that Lilly was a horror at the time. So it must have happened in the last 10 years.
We know that Lilly often messed with Shannon or Frank. Therefore, I assume she was up to something that would have resulted in a punishment from Shannon that would have been much more draconian than what Becca did, or it must have happened about 5 years ago when Becca started to assume that Frank was her father. I suppose it would have been something that could have gotten Frank in trouble or hurt him a lot. Therefore, this would have potentially been the first time Becca was forced to intervene on Frank's behalf.
If this last theory is true, then I hope we learn more about it in Becca's story. It probably wouldn't change my opinion of Lilly, but it would be interesting to see what Lilly's modus operandi against Frank was back then.