I realize that I do tend to rattle on about this tendency that the Pattersons have of not seeing that other people have as much right to their little treasures as they themselves do but I think that it's fairly important to understand why. The first hint to as to why Elly felt the need to swoop down every so often and clean out Mike's room doesn't have a thing to do with helping Michael develop a sense of pride in his surroundings. As this strip indicates, most of why she feels the need to decide who owns what is that she fears censure from other people more than she values the opinion of people who get in the way of trying to impress people who don't actually count. Simply put, Michael cannot have an identity separate from hers because it gets in the way of reassuring the disinterested that she can be relied on. The idea that no one anywhere has ever been worried about her reliability is not one a silly, attention-seeking dimwit like her has ever contemplated. Always and ever, the need to be seen as being someone who can do things for people gets in the way of respecting people who actually need her.
We must also remember that she simply isn't capable of caring about the rights of those around her to do what they want with their belongings any more than she can understand who owns what. In both of the strips I linked to, April's life is made the worse because her idiot mother and father simply can't be asked to understand that if something belongs to someone else, it's not junk that THEY can dispose of how they see fit. Sadly, this need to think of having to respect the rights of other people as some sort of horrible imposition is a collective failure of the imagination.
My guess is that it comes from Lynn herself. As we're about to see in the notes, she quickly grew tired of her mother-in-law's need to simply toss things that meant something to her in the trash because since they meant nothing to Lynn, they couldn't be said to have any value at all.