I'm sorry to hear that your spouse's adult daughters have not been supportive or compassionate about her mental health. One thing that I really took from your post is how much you care about your spouse, you must be a great support system for her.
I found this link from the Beyond Blue website which provides some descriptive phrases of depression or how someone feels when they are depressed : https://www.beyondblue.org.au/get-support/have-the-conversation/talk-about-it
I have heard from others that showing videos of people explaining their story of depression is both informative and interesting which may be key to getting people who are not compassionate to understand.
On Youtube if you search 'my story of depression' there are some vulnerable and informative short videos which you might find helpful to show them! Also there are some Ted Talks about mental health which may be useful (however these tend to run for about 20-30 minutes). Here is the link: https://www.ted.com/talks
You can then search 'depression' or 'mental health' and see if you find any talks or videos you connect with that may be helpful to show your stepdaughters.
If you would like we can find some fact sheets or websites that may be useful, please let us know if you need help with this.
I hope that this helps!