I'd try to find the new mom a private room for a while. Even if the other cat doesn't bother the kittens, the mom cat will likely be very touchy about everything and get nervous about having any other cat around. The less stressed she is the more she can concentrate on caring for the kittens. (She's also likely to hide herself and the kittens if she feels threatened.)
You could let the other cat visit the kittens supervised. I doubt a female cat would harm the kittens. Actually most adult cats are very careful about kittens.
I would allow them together supervised. There was a study recently that showed feral cats in colonies help care for each other's kittens, much as lions do. I think it's possible the same would happen in a home, if the 2 females get along. It would be interesting to observe...
I just brought in 2 five week old kittens. My 10lb 12month old female cat hissess, growls and attempt to swat at them through the tiny windows in the box where I keep the kittens. I suggest you keep the kittens away from any other cats.