It's funny the connections our brains make... my sisters are 15 and 17 years younger than I. For years, they were "the kids" to me. Until I had my second child.
One child, no problem. Two? Brain wiring went cuckoo and I called my kids by my sisters' names and vice versa. My wiring for "the kids" got crossed when I had another similar grouping in my head.
I find it a fascinating reminder of how our brains make connections, and the one time my partner called me by his OSO's name (although there have been other "oops, wrong girl" moments) it's more of a reminder that we're both close to him, as opposed to feeling like he's thinking of someone else. But then, I can be a bit of a psych geek and eat this stuff up.