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I would like to help you with your question.
I can understand how upsetting this would be for you. Certainly, your 15 year olds statement is inappropriate and leaves you wondering about his sexual desires. What is needed right now is a serious conversation about what he meant by "liking" you and a chat about sexuality. Has he had sex education classes in school? Have you or your partner talked to him about this? It's hard to know if he is just confused, if he was joking to get a reaction out of you, or if he has sexual feelings for you.
Do you know if he has had any prior sexual experiences? Has there been any incidences of inappropriateness?
For the time being, it probably is better that he not be alone with his sister until you have a better understanding of this situation.
However, I want to caution you to be very, very careful in how you talk to him. The one thing you desperately need is for him to be open and honest about what he is thinking and feeling. Thus your approach has to be one of understanding, sensitivity, respect, calmness, and compassion. The last thing you want is for him to feel accused of something and then for him to get defensive. What you are aiming for is an open discussion so that he can appropriately direct his sexual desires.