There's no direct relationship between the two currently. They share a lot of underlying technology but Filecoin doesn't directly incentivize IPFS, and IPFS itself isn't directly used by Filecoin either.
The basic reason why is because the protocol to request a file from Filecoin is a lot more complicated than simply fetching a URL, so IPFS can't simply sit atop, at least not without other changes.
The basic reason why is because the protocol to request a file from Filecoin is a lot more complicated than simply fetching a URL, so IPFS can't simply sit atop, at least not without other changes.