Well, since you asked.....
I see nothing wrong with any sexual activity between consenting adults. If siblings want to have sex, and they are both adults and both are truly participating of their own free will, there's nothing wrong with it.
On the other hand, the behavior you describe is bizarre, unless you had earlier given some very strong signal of interest yourself which you don't mention. Moral objections have no place here, but caution does.
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Coito ergo sum
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