My mother understands that it's something important to me, and also understands that it isn't just a simple line (since otherwise I wouldn't be interested, she knows my tastes well). She understands that it has a complex lore and plot, and enjoys knowing that I utilize my creativity through it. I grew up with BIONICLE for as far back as I can remember, I don't remember asking for Kopaka Nuva or Pahrak, but I know that I had them growing up. As far as I know, she got me into BIONICLE in the first place.
My father also likes it when I MOC. He doesn't know much about the story, and I don't usually talk about it around him, but he knows it's important to me, and respects it, much like my mother. Neither parent cares that I spend money on LEGO, and, funnily enough, when my mother heard G2 had ended she asked if I had gotten any of the 2016 sets before it did, seeming almost concerned that I wasn't able to pick them up. It's kinda funny, actually.
Yeah, I'm pretty lucky to have cool parents, I know not everyone has parents that are quite so... accepting. I have far too many friends with dictators for parents.