Maybe the slider would work like this:
1 fertility = up to 17 percent
2 fertility = 34%
3 fertility = 51%
4 fertility = 68%
5 fertility = 85%
6 fertility = 100%
The idea of litters would be difficult, as mentioned in other comments, nest size. though I do think if one sibling was smaller, like a runt of the litter, 3 may fit.
Or maybe you could 'expand' a nest, by making 3 other nests around a naturally spawning one, in a relatively flat area, you could merge them and make a large nest (taking up 4 tiles) you could have 2 mothers sitting on it. This big nest is permanent.