Consider this...A fifteen-year old who may be trying to find her own way while her father who is sixty-three years her senior is "losing it."
Your husband who was the pillar of the community or at least his family is now vividly aware of what he can NO LONGER DO. This is understandably depressing and scary. For all you know, he may fear that you will leave him.
Meanwhile, you're in the middle trying to survive and find some peace in your home.
Beyond what Frena has suggested earlier, you might consider making this a fun challenge and life learning experience for your daughter. Both of you can undertake to learn as much as you can about your husband's dementia and then explain to your daughter that if she can succeed in communicating with her father, she will learn life skills that can help her with her friends, at school, and at a job. (Please note, I don't have teenage children. Yet depending on the approach, it may just work!)
For whatever it's worth, my father was forty-nine when I was born. And back then, most parents were in their late teens or early twenties when they had children. I had to deal with, "Is that your grandfather?" "No, he's my father." "He is?"