I think this is what happens, though. If you have a particularly talented member of the family, they get special treatment to encourage that outstanding talent. Unless your MIL can afford to pay fees for all of them, then it's not unreasonable of her to dish out her resources on a child who has outstanding talent.
I'm sure the rest of the gcs are wonderful and lovely children, and are treasured and loved, each for their own specialness, but a talent which is recognised by the world, society as it is, (and can therefore be encouraged), well, it's lovely that MIL wants to do that.
Maybe if your child excels sufficiently at sport MIL will offer to send her somewhere where that will be nurtured too. You don't want her to go to a special sports school though (does such a thing even exist?), you just want her to go to a special school, just because it's better than the school she's currently at, not because everything will be built around her special skill. That's just jealousy, really. Understandable, but not a thing you should give into.
Be glad that someone in your family has that sort of talent and is lucky enough to have it fostered. Maybe one day, your own child will do something outstanding enough, that your MIL will offer to pay for special coaching for her.