Your entire second paragraph is the definition of "white guilt". You didn't even know about the racist stereotype, held no ill-will, and are still apologizing. You have nothing to apologize for. It's about intention. You were just bringing things that can be enjoyed for the meal. Like for fucks sake why do you think it became a stereotype to begin with... it tastes good!

Your family was being hypersensitive and subconsciously made it racist because of their associations. You didn't do anything wrong. Are you just not allowed to enjoy fried chicken and watermelon because someone might be offended? If they are, tell them you're sorry and you just enjoy them together. They're welcome to do the same. If they want to assume you're racist just because you're a different ethnicity doesn't that honestly make them the racist?

These people are definitely racist shitheads though. The intention was obviously to mock the holiday and get a reaction out of the one person they invited, but it's just foolish for you to take blame for something other people find unintentionally racist. Just enjoy different cultures with respect and be inclusive and you aren't racist.