The main reason that they don't is that they know that she never seems to know what's going on around her. As by way of example, let's examine some of the stupider things that she did to make the Housening even more annoying than it already was. In the first instance, a very stupid woman stupidly assumed that since the clutter she was complaining about was in April's living space, it belonged to April instead of making the association anyone else would make. In the second instance, the same dull mind didn't realize she was dealing with a fait accompli. While I do love to pillory John for seeing what's going on around him and willfully misinterpreting it so as to cast April in the worst possible light, at least he's paying attention to his surroundings. Elly lives her life in a self-induced fog and is thus a worse person than someone who misuses his awareness.
The first strip I linked to takes me to the second reason why she's lousy at giving advice to other people: a sort of tone-deafness to their concerns. While it is true that April's belief that she was being shoved in a dungeon were a tad overblown and that moving wasn't really the end of the world, she wasn't exaggerating by much. What she had to deal with was a pompous ass of a father who'd made his mind up about what she was angry about, a whining infant of an older sister who acted like a dog in a God-damned manger, a spoiled brat older brother, a sister-in-law who saw her as free labor and a mother who couldn't be asked to know what was going on around her and no one wanting to take her seriously. Elly's response to the problem of clutter is not to realize that April felt like just another piece of flotsam no one knew what to do with but to ask her why she didn't tidy up.
Another reason that she's not good at giving advice is that she usually has the ulterior motive of making her own life easier in mind when giving it; what's more, the people around her know it. She might boast about her fairness and impartiality and willingness to see their side but at the end of the day, it's "do what makes my life simpler" instead of "do what's best for you." No one in the world wants to take the advice of some smug boomer numbskull whose guiding philosophy is "Since we did you the favor of feeding, housing and clothing your freeloading five year old ass when you were faking being dependent to ruin our lives, you owe us the favor of supporting us in the lifestyle of waste to which we've become accustomed despite our having spending habits that would alarm a heroin addict."