That depends. How hot are they?
I ask this because those of us who are mathematically inclined are familiar with the Strauss-Klein-Bernstein equation. The outcome is a number which can be compared against the subject's Tesla-Jordan coefficient (colloquially known as hotness) in order to determine if dating this person is appropriate. Let me show you the equation and I can break it down:
Let H prime be the threshold for the Tesla-Jordan value, and we all know it's a function of Psi, but as we know, Psi is also a function of age, denoted by r, and t, closeness between your sibling and the person in question. Of course, there's a problem, H prime is normalized for Western-European societal conditions, so you have to compute the proper normalization for American society (which is represented by the expression that isn't a function of Psi, as you can clearly see). Naturally, we need to take the appropriate delta values, compute h hat, divide by the permittivity of free relationships, and then V(r) naturally becomes the "market value." The point is, when you're done, you end up with the expression on the right, which -- when you plug in all of the right numbers -- you learn that the proper Tesla-Jordan value is any number in excess of '7.4' on the Albert scale (standard 1 to 10 measure of hotness).
Of course, you could also talk to your sibling and ask if it's okay. Communication is key, that and taking things situationally. Unfortunately, human beings don't have a set of hard and fast rules on stuff like this -- unless you want to default to Strauss-Klein-Bernstein calculations on the fly. In which case, I must suggest you carry a slide rule with you at all times.