On his weekends, then you might have more of an issue.
You can send some product with your daughter to his house, but that still won't guarantee that your SIL will use it. I am not sure that there is any way around that, other than your ex stepping up and telling his sister to not wash their hair using her cr@p.
if you're not there, and it's not your weekend, then I really don't know how you'd address it. Even if you get mad, talk to her, talk to him, send product...it still sounds like she's going to do what she wants...if not even do it MORE because she knows it upsets you.
I don't know that you could maybe get your daughter more involved, maybe she could tell her aunt that she likes your shampoo better and to please not use her stuff on her hair? Maybe make it look as though it's your daughter's idea, not yours? (I do this with my kids, "Hey, next time grandma asks you about blah blah blah, please tell her blah blah blah." and they do, and they don't mention that I told them to tell her that). Maybe your daughter can communicate that she doesn't liek the pressing, and likes her hair better when it's dried naturally?
I am sure that there is a way around this, but with the dislike that SIL has for you, I am nto sure how you'd address it without making it worse!