Our older cat (3 1/2), has turned against our younger cat, 2. This seems to have escalated when younger cat was in heat (before being belatedly fixed). Both are female (we have no other pets), and both are indoor cats. Older cat now avoids younger cat, hisses/growls at her, and has begun peeing on carpets rather than using litter box. We're on verge of having her put down or setting her free outside. Oy! Any brilliant suggestions?