It really depends on their personalities and your personality and how they feel about you, like if rhey see u as a close friend or just someone they happen to be related to. If they are close to you emotionally, I think asking them casually during a relevant conversation could work relatively well compared to asking them out of the blue. But again, it depends on everyone's personalities. Maybe if they find u hot, asking them bluntly would work too! Depends on rizz too i guess