It could be...
...that she felt you more deserving than they. You can't choose your family, but you can choose not to let them get their ungrateful mitts on an undeserved inheritance.

Also, some people have a knack; of sizing up people they've just met with uncanny accuracy. I can know people for years, but still not figure what makes them tick. On the other hand, I know people capable of instant telepathic-grade character analysis.

Perhaps she was planning to move into a residential care facility, and couldn't take them with her.

There's any number of possible reasons, but she sounds like she was of sound mind (mere playfulness does not correlate to mental incapacity); and they were hers to do with as she wished. So, you now have something really nice to remember her by. And finally, in my experience, it's only those who *don't* question such events, that are undeserving to receive them.
(The Incredible Sulk Finally remembered his password on, Thu 6 Nov 2008, 6:26, Reply)