The following things have been going through my mind over the last few days:
Abby's confrontation with her biological father: I don't think she had any doubts about Frank's statements or that her mother told Frank a wild story that he ate up. I think on the one hand she wanted to make it clear to him what she thought of him, also for her mother and Frank. On the other hand, I can also imagine that she wanted to know whether it was the whole truth, that there were possibly things that Jess couldn't know. Because I think she would have made a difference if it had gone like this: Aaron's parents are wealthy and find out that their son has got his girlfriend pregnant, the parents don't want a scandal and force Aaron to persuade Jess to have an abortion. Jess refuses and Aaron comes up with a solution: he wants to take responsibility and at least support her financially. His clique fears that they will lose their buddy and start these rumors, claiming that Aaron is also involved in these rumors. Jess drops out of college and Aaron can't tell Jess that he wants to support her. Had the truth played out this way, I think Abby would have at least had less resentment towards her father. She would never have seen him as a father but would have forgiven him once she had overcome everything and had no problems meeting her half-siblings. But since he only revealed the most necessary things to his wife himself, that would have been very implausible after her “Wha...?”, even if he had claimed it. But I think that she will meet her siblings at least once when she has come to terms with it. I also think that even Jess would advise her to do so, as a core message of this story and its actors is: Children are not responsible for the actions of their parents.
And I think that Lilly and Frank are more similar in certain character traits than they realize.
Both had a severe trauma in the past that was never properly dealt with.
Both have a close friendship with certain redheads with whom they should become best friends and more.
Both would do anything for their best friend to help her.
Both can be quite stubborn. Frank who never gave up on being a father figure to Lilly and Lilly who refused even though Abby and Becca talked her into it and hoped for her father instead.
While neither are stupid, they are played like a fiddle by more manipulative characters (Becca, Shannon).
Both have held on to impossible family images (Frank with Shannon, Lilly with Trevor).
I don't know if it counts as irony, but Frank's worry of rejection/getting it wrong, which is why he never actively tried to start a romance with Jess before Abby, is mirrored for me in the situation where Lilly tried to start a definite relationship with Abby, where she was turned down because, if I recall correctly, Abby thought Lilly was joking.