LG, this situation does seem like an opportunity to meditate on what you can control and what you can't. It doesn't have to be control or not--you can focus on controlling selectively what is yours to control.
Honestly, your older daughter's situation strikes a chord for me because I suffered through life with an alcoholic/rageaholic father and I can only imagine what she's going through with your ex. I hear what you're saying that you've been trying to preserve her relationship with someone she once considered a father figure, but it sounds like she's been pretty clear that she doesn't enjoy visiting him anymore.
And I can understand that you're concerned about your younger daughter who is obligated to go. Sounds like she needs some protection from you, too, in ways that are open to you, whether it's documenting the abuse she suffers at her father's house and eventually trying to get supervised visitation, or making sure she has access to a phone and can call anytime she feels uncomfortable, or one of the many other coping strategies I've seen parents using here re: visitation with an addict ex.
I can't offer any ESH of my own on dealing with relationships between children and A parents/step-parents because I've never been in that situation, but I've read the stories of so many parents on here who have put so much time/effort into getting legal permission to cut back on visits between their children and addict exes in situations where the ex was the natural parent and had a legal claim to visitation. It occurs to me that you're maybe fortunate in that you don't have to go through that to protect your older daughter from your ex--this is something totally under your control. Just my $.02. I'm wishing peace and serenity for your and your kids.