Okay, this is a funny one. I was talking to a person on the phone about a potential dinner and I had to tell the person that it was going to be for kith and kin only. Now, I was real tired when I said that or otherwise I probably would not have worded it that way because the person was like, "What's a kith?". LOL
So for those who aren't use to Southern slang, kin are your blood relatives. Kith are the other people who are close to you, friends, neighbors, church members, whatever, the people you have a close bond with but aren't actually related to them. So, if a function is for kith and kin only, that means that no strangers can come and nobody can be bringing anyone whom the family doesn't know.