If your sister is old enough to have a husband and kids, her health care choices aren't any of your dad's business. Believe me, I understand domineering parents, but seriously -- she's an adult and it's up to her to make the decisions she makes, particularly about whether to take psych meds. That might be something you two want to stand united on against him -- she clearly needs help, definitely therapy and probably meds to get over the hump.

Keep us posted as to how it goes.

Candy