My cat had kittens and my other female cat did EXACTLY the same thing. I mean, it got to the point where I couldn't even let her near the kittens. My hubby and I moved out of state when the kittens were 2 weeks old(eyes just opening) and the whole ride(about 11 hours) I held the kittens in my lap. (Don't worry, I brought mama up to feed them every 2 hours). However, every 2 minutes "Auntie Estrella" would try and climb in my lap and nurse the kittens! It drove me up the walls! Later, after we moved, the mam cat would sit out and watch her kittens as they learned to walk and play and while she was there the other cat would come over and attacke her becaues she was trying to protect "her" kittens. It got to the point where I would have to keep the crazy "auntie" in a crate almost all day because otherwise she would get it the bed with the kittens while the mama cat was gone and wouldn't let mama back in to feed the kittens. Obviously, I didn't want the kittens starving to death so I had to crate Estrella til the kittens were old enough to eat solid food.
I just found homes for the kittens about a weeks ago. They were 14 weeks old and let me tell you, Estrella was their favorite aunt in the whole world! While mama wouldn't let them nurse, Auntie Estrella would gladly flop over and let them suck away to their hearts content(even though she doesn't have milk!).
I have no advice because I couldn't solve the problem myself. I just recommed keeping them seperate til the kittens are old enough to eat solid food, otherwise they may not get what they need and could die. You could also use a spray bottle and spritz(careful not to spray near the face, water could get in her ears) her whenever she gets near the kittens. Eventually she will learn to stay away from them(at least while you are around). I did that and it worked, but only while I was there. If I wasn't there I would have to put her in a crate. After a couple spritz though she learned to get away if I just said her name really mean or shook the bottle.
Best of luck to you, I know how hard it is.