I won't talk about same foci bonus anymore then. Let's assume we are talking of situations that you have different foci.

But you don't choose forms yourself. If you have a set of forms of different families, You'll probably have some good forms to some of (lag, resist poison, regen, water breath, cause bleed, dodge good, strong carapace, ...) But what if you are not lucky enough to have different family forms? (Once I had ocelot,tiger,snow leopard,clouded leopard) You have some forms of same family, with almost same capabilities and lacks/weaknesses. If you get a bonus just because you have same family forms, It'll balance it somehow.

As others said, give lower forms more utility/additional capabilities. These capabilities can be very weak at first, but they get better as you learn a higher tier form. So for example, you will have a tier 4 form that can do a trick with 10% power when you get it (when form is confident), 25% when you learn a tier 3 form of the same path, 50% when you learn a tier 2 form and 85% when you learn a tier 1, but if a tier 1 form has the same capability, it'll have it at 100%. (100% doesn't mean always success as you all know, it means the best as the form might know.)