Blimey, OP. No wonder you’re stressed.
Why won’t your DD apologise? If she’s admitted she said it in retaliation to being told off.
You need a serious talk with her. On the one hand, she needs to know that if anyone does anything inappropriate to her, her sibling or another child, you’ll always believe her and so it’s very important she should tell you, as that’s a crime. On the other hand, you should explain that’s why it’s very serious to cry wolf and say it about someone if it’s not true. Because it’s accusing someone of a crime.
And finally, you need to explain to her (in an age-appropriate way) how calling gay men pedophiles is a long-standing homophobic trope. So, she should challenge people who she hears say that. And that gay men, like her uncle, used to be at risk of prison simply for being gay until 1967 - and 1990s in the military.
Hopefully, she’ll realise why her uncle is so upset. And hopefully, if she apologises to him, he’ll understand she’s a child ignorantly repeating things she’s heard and now understands and is sorry. If he holds a grudge, that’s on him. But TBF I wouldn’t blame him for not wanting to look after your DC alone.