Sad situation.
This isn’t about horses really. This would happen with any job that requires hours like this. That may be how you have to approach it with her: this is NOT about horses. She would have this problem if she was a lawyer working 16 hours a day, 7 days a week.
Suggest your sister go back to counseling FOR HER, in addition to couples therapy. She needs to see that this, in part, due to the career choices she’s making, and she’s going to have this problem over and over again. She needs to at least be aware that this is a choice she’s making, and that it’s always going to have this outcome, no matter who she is with or what she’s doing. A therapist can help her sort out what she really wants, and help her to see that if she continues doing things like this she’s going to have to sacrifice elsewhere.
The only way around it is if her husband works with her, or equally brutal hours. But one partner being emotionally/physically/mentally unavailable pretty much everyday/every night simply won’t work.