Is it worth the risk to apply to REUs? How likely am I to be accepted?
I don't know where you are getting the idea that it is a longshot specifically for non-minorities. In general, REUs do have low acceptance rates for everyone. There are a lot of students who want to do them and not that many REUs. 

Now, to answer your question: yes, being female will likely help somewhat, but it likely won't make a big difference. What will matter more is the GPA which will help out a fair bit. The previous research experience may hurt or help; at least in math some REUs try to admit undergrads without experience to spread them around, but that's not all of them  by any means, and for the most part it would be helpful. I suspect the situation in biology is similar.

Since you are a junior, you should already have some connections within the biology department, either your adviser or faculty whose classes you have taken. They can likely give you more personal advice since they know you better. But at least from what you've said to us, I suspect you have a higher chance of getting admitted to an REU than the average candidate.