Or the parents say, "Oh, it's too bad that all that stuff is taking up space in the garage. Let's see what we can do. That's a 3-bedroom house. The upper right bedroom is not needed--so don't put any furniture in it. We'll keep that room as ours, and not yours. After all, you're not paying any rent, so we can't really let you use the whole house. However, you may move all that stuff from the garage into the bedroom, so that you may have more room in the garage. Then close the bedroom door, and don't think about it again."I would just put locks on the garage. Take the garage away from the SS sister.
The OP & family are halfway across the country or something--they'd have to get someone to do that for them, and there's no way to be sure she wouldn't get a crowbar and pry the hasp off.
And I second Hillia's nervousness.
That's one reason why I suggested that Mom say, "just move it into that extra bedroom," to redirect Sis. Heck, it might be easier and actually effective to hire someone to move it into a spare bedroom; then you know it's in there, and there's no reason for Sis to keep seeing it and having it bug her.
I don't know if the stuff in the garage is really in the way--it may be a territorial thing on her part. But even if it is, moving it all into the smallest bedroom might remove the "reminder" part.
The other option might be to move it all into a storage unit, and make Sis pay rent to Mom & Dad that's equal to the storage-unit rent. (I wouldn't want her paying it directly; she could just stop, and the stuff would be seized.)