Caregiver taking advantage of my step father. What can I do?

It appears you're doing all you can do. It isn't uncommon for the elderly to be hoodwinked and then refuse to take action when they discover the truth. They're embarrassed and usually don't have the strength, physically or mentally, to fight. I'm glad you have taken on that battle for your stepfather. This woman sounds less than stellar from your description. Were you able to have your stepfather change his will to exclude this woman? If not, it appears the damage has been done and the only thing you can do is watch her like a hawk. What an unfortunate mess! My mom befriended her caregiver and in the end, I was excluded from her will and said caregiver was added to the will. My mom didn't have dementia so according to my lawyer it was all legal. I didn't want or expect anything. It just hurt to see that my mom wanted everyone to know I never existed to her.