I am a hoarder, and the daughter of a squalor hoarder. It's painful just typing it.
You might not guess from my home. It’s clean, normal looking,
and relatively organized. The 'home' I grew up in, was very extreme and the total opposite, so for a long time I was in happy denial that I was a hoarder, because I presented so very differently from my parent.
I’ve spent years battling myself and eating away at erasing and controlling different parts of the condition. The stuff is just the visible symptom. It absolutely is a MH condition, and genuine educated help is rare.
Part of my issues may be learned behavior, and part may be genetic, but a large part is almost certainly a response to a very horrible shameful childhood which included the repeated loss of everything, and everyone, combined with a creative ability to see potential in lots of things that others wouldn’t.
The clutter scales cause a lot of issues for people who are in control of their possession, homes, and cleanliness, or who are additionally hoarding in storage out buildings etc, because the images don’t relate to how many live.
Additionally you have a raft of people living in extreme foul conditions, considered hoarders who aren't actually hoarders, having no emotional attachment to what they're surrounded by.
When is it an issue? When it damages and impacts yours and others quality of life, and/or mental well being, use of space, and finances. The bigger the home and or additional storage, the more potential of ‘getting away’ with hoarding tendencies, the smaller the home the more likely you are for it to become problematic earlier.
If you’re sentimental and keep everything from childhood, etc, and have the room to comfortably do that, you’re actually fine as long as that situation remains.
The problem comes when you don’t, especially if life changes suddenly, and you find the impossibility of letting go is greater than the need to, regardless of the price of that.
The reason your cousin is so upset is because stupidly we stigmatize people with the condition and treat them as lesser with lesser rights.
While items are kept tidily, displayed, organized and clean, it is technically labeled a collection, only the owner knows how deep or problematic or not their attachment is, and if it's actually organized hoarding.