Can you still sickle cell if you're >3/4 white?

Yes. If you have true SS disease you inherit a defective sickle cell gene from both parents. That means they both carried one defective gene ( in a pair tasked with making proper hemoglobin). It doesn't matter how far back in their family tree it started, or that it ever showed up before.
Yes. While sickle cell disease is most common in those of african, or african-american descent, one can still acquire the disease genetically based on their heredity.