Strange

All right, around the beginning of February my brother's cat gave birth to five kittens. Over the past few days, there's been an odd behavior pattern among them that I can't think of a source for. Whenever I'm alone in the house, or the only person up, there will be a kitten next to me. It's not the same one all the time, they seem to be taking turns. If someone gets up, or someone comes home, whichever kitten is with me will split, and I won't see it again for about half an hour--and then it's usually in the company of another kitten.  (Whatever this is, their mother doesn't seem to be apart of it.)

What is this? Why is it happening, and should I be concerned?

It sounds like they are keeping you company!  They don't want you to be alone!    They sound sweet; I wouldn't worry at all!