As I've said before and yorba reminded me, April has no real concept of her family's history. She's been told certain things but not in a systematic way that would allow her to figure out why her parents and siblings act like they do. The case in point I like to use is her use of Liz's hated nickname, "Lizardbreath". She doesn't know why it enrages her older sister because she has no real idea what went on because people around her keep changing the subject. She doesn't realize how hateful and vicious Michael used to be or how impossible it was for Liz to defend herself against his unwarranted attacks. Without that knowledge, all she sees is a big sister with a humor deficit. Similarly, she has no idea how stubbornly John fought to keep Elly from having any sort of job nor how patronizing a dick he was. Not only does she not know why her mother is so resentful all the time, she has no clue that she's affected by it. She may realize deep down she gets treated badly because she's in the way of what they want to do but she doesn't know that her mother halfway thinks John oopsed her into existence as a means of enforcing his will. By keeping her in the dark about their past, they've made sure that she'll always say and do things that bother them.