You have had the cats for 4 years - is that correct? And this problem started 6 months after you got them - is that correct?

So this problem has existed for 3 1/2 years, if my math is correct.

Why did you wait so long to find a solution?

I also recommend you get all of them to the Vet as soon as possible. Get that female fixed - that alone might solve the problem. However, since this has been going on so long, it's probably just second nature to them now.