First, I hope your daughters are finding the support and resources they need for their mental health. It is a difficult time to be BIPOC in the U.S., let alone biracial or multiracial. Add to that the "standard" challenges of being a teen (e.g., figuring out your identity and who you want to be), and you certainly can have unanswered questions and stressful challenges.
I don't know what the right approach is, but my parents always emphasized our history and tried to instill pride in our racial and ethnic heritage, so my brother and I might have some sort of "shield" when attacked or shamed for our differences.
As biracial individuals, your teens are dealing with additional layers of complexity and parents who don't quite understand the same experience (not necessarily you, but it sounds like their white dad isn't as comfortable talking about racial identity, let alone biracial identity). I know there are some academic resources about being biracial or multiracial, as I took a course on this 20 years ago when I was an Asian American Studies minor. It was mostly academic scholarship, not written for casual reading, but I found it very eye-opening and insightful.
I don't have that syllabus anymore, but I know many biracial and multiracial scholars would share their own experiences and then supplement with additional field studies to draw bigger conclusions. I just tried googling and found this article where the author does just that (sharing a little of her personal experience but also reviewing the existing academic scholarship on multiracial and biracial students' educational experiences). Perhaps finding more info like this might help your daughters? https://digscholarship.unco.edu/cgi/viewcontent.cgi?article=1079&context=jeri
The article specifically talks about how some "monoracial" individuals try to gaslight multiracial and biracial people and downplay their unique experiences. Sometimes seeing an acknowledgment of what you've experienced in writing is validating. I send my best to you and your family.