People, especially women, died quite frequently as young adults.
You say you don't have evidence to contradict her own statement about who her eldest son is.
It seems that, per the best available evidence, she is not his mother. I don't think she should be set as his mother until the best available evidence suggests that she is. I don't understand why your desire to keep her as an ancestor should trump what the record reflects.
P.S. just this very morning I ran across a case of this in my tree, and am working to have the mother removed from the earlier children of one of my ancestors. Do I want more ancestors??? Of course I do. But I don't want to latch on to people who aren't my ancestors, and I don't want to perpetuate information that is contradicted by primary contemporaneous records. I hope wikitree is not becoming a place where this is the way we lean, because I would feel compelled to keep arguing about it each time it occurs in my tree.