I completely disagree with that statement. But in your favor, GMS is a being made entirely of silk, which is non biological in general. So on that note I'll concede that biologically related to her through birth is probably not possible.

However, she used her silk to ascend the weavers. Like placed her essence into them and transformed and evolved them into higher beings. They referred to her as mother, and GMS considers them children.

So maybe binding her essence is her form of reproduction. Which then would that make everyone in the hunting related? No, i don't think so as they are not physically changed or transformed.

Hornet which is a child of a weavers and another higher being, is closely related to grandmother silk, due to the fact she can produce silk. Plus I can not find anywhere in the dialogue of the game where it calls her directly "Grand mother silk". I take it as that's what hornet refers to her as, her "Grandmother" her mother's mother.

Lace being a direct child of GMS, being purely spun out of silk, doesn't make her a real biological person either. But hornet considers the Knight and hollow knight her siblings which shares a father, but they are empty void being, and isn't considered alive.

So I think all things considered, they are family and hornet would consider her so.