Kittens inbred.......help?

Emily, good for you for rescuing. You are right to be concerned about health issues related to inbreeding. That the kittens have survived for three weeks is a good start, but no guarantee. I would urge you to have your vet do a "once over lightly" to be sure that these kittens will be okay. That way you will know if you have any heart or internal organ issues. Neurological issues may take awhile to manifest. He/she can also give you advice on the runt. Runts can go either way. Some don't make it; some do. Some stay small and some eventually grow to match their siblings. Standard spay/neuter advice applies. purrs