It's interesting to read the replies. I think the mom in question handled it terribly, but I DO think that the mom should step in. I don't think that a 5-year old necessarily knows how to interact, and I also don't think that an older sibling who picks up a ball lying around should be forced into trying to "work it out" (what's to work out?). I would have stepped in to say briefly, "You weren't using this. There's no good reason that your sister can't use it for baseball. If you can't respond without yelling or whining, you need to go to your room until you calm down." End of discussion. In our house, there are no toys that are off limits to siblings (and I have four kids). It's just weird to me. It's never come up, honestly, never in all our years (and two are grown). It's not a rule; my kids just aren't that possessive. My boys share a room and a playroom, so they pretty much play together. We had this issue with the girls and makeup/clothes as teenagers, and then we set rules - and we had to get pretty specific (different rules for consumables like makeup and checking first that jewelry/clothes won't be worn the next day by the owner). Overall though, I think the good part of having four kids is that they pretty much learn early to share. And I don't put up with whining and I don't put up with it's not fair. We run a commie household- kids get what they need (not what their sibling gets) and they help as they are able (and if their sibling is doing homework and can't do dishes, sorry kiddo, you're up).
I'm a total AP mom - I nursed longterm, I coslept, I don't use corporal punishment, but this isn't about AP, this is about whiny, indulged kids. They are going to have a very hard time in school one day, not to mention in the work force!
