This one is a kind of weird question to answer for me because it varies so much. I ended up picking one of the unsupportive answers because it has the most affect on me.
My boyfriend is completely supportive of all my collections (MLP, toys, and dolls). He didn't like when the pony display was a chaotic mess because I had no way to organize them properly, but I didn't like that either. Now that I have actual furniture and not just boxes everything is much nicer so he doesn't mind at all. He doesn't mind any of the stuff I keep around my computer in the living room either, and if we had a say in decor I doubt he'd mind some stuff chilling on display in the living room. He's bought ponies and dolls, including extremely expensive ones, for me as gifts. He even has a couple of his own, like a custom Jack Skellington pony and a SHG Harley.
My basically MIL though.... she's very.... weird. She's supportive in the sense that she'll buy me pony/doll things for holidays, but in general she's extremely unsupportive. Any time I acquire a new possession, even a 1in tall figure, gets a lot of complaints about "omg where will you put it?!?!? there's no room!!!!!!!" when she'd never notice if the figure was on boyfriend and I's bookshelf or not. And she's tried to pressure me to sell them in the past. But with the very expensive doll I got for my birthday, she fawned over all the very expensive clothes and was saying how cute they all were. She was very encouraging about (eventually!) having them all, and she said she'd buy a really really overpriced piece for Christmas so I was stunned. Aside from that, she actively discourages my collections.
She just really really hates actual, tangible objects I guess? She doesn't mind my boyfriend's video game collection (on Steam) in any way and is very encouraging of him buying more so its very weird. Its completely backwards from anyone I've ever known, especially older people.
The rest of the family is mostly supportive!
My boyfriend's uncle buys me pony or doll things, he doesn't even care. He'd buy whatever I put on my Christmas/birthday list as long as it was a reasonable price. He's never said anything about it either way though really? So he's like, very chill supportive.
My aunt is super supportive! One time when I went down to visit we all (aunt, cousin, granny, aunt's friend) went to a flea market and every single person was on the lookout for ponies :D She was really excited when we found some. On another trip she bought me a G4 when we went to Target to pick up some groceries. She probably would've bought all of them but I was too chicken to let her! She sent one of those hand airbrushed t-shirts from like, Fairs and stuff a few years back for my birthday. It has a generic pink G3 and my name on it. This year she found someone locally to paint some fakies for me! And when she found out about my expensive doll she was really excited for me. So she's just amazing.
My granny is also supportive! She's passively collected dolls for basically her whole life and enters an AG-size doll outfit in the Fair most years. She has a small collection, mostly from other countries in traditional outfits, and she seemed really relieved when I ended up liking dolls too so they'd have someone to love them. She's bought me a couple things here and there, but mostly she buys me the practical gifts I need. I talked to her a lot about the expensive doll because I was excited to share it with her, but she thought I was hinting x.x So I can't really gush about them to her I guess cause she took it the wrong way!
My great aunt (granny's sister) is also very supportive, at least of the dolls. She was also really relieved and gave me her childhood doll collection last time we visited. She doesn't buy gifts really, but she collects both dolls and teddy bears so I think she gets it and doesn't mind my ponies either.
For the most part, my close friends are collectors or at least doll/pony admirers too so they get it :) Those that aren't don't really seem to mind, although I wouldn't call all of them supportive really? Mostly they're like, happy, polite neutral.