Feathertail is related to Mistystar, Mosskit and Stonefur.

This is because Oakheart is the brother of Feathertail and Stormfur's grandfather, Crookedstar.

Because of this, and the fact that Oakheart's children are Mistystar, Stonefur, and Mosskit that makes them great-cousins...

Or something.

I don't know.