How do I interpret it!?

sweetie, this guy is so obviously flirting with you. very very very obviously. getting into bed with you unnecessarily. steeling away with you while the rest of the family is playing together. making his computer tell you y jokes. and asking you questions in order to show interest in you (even though he already knows the answers to the questions). these are all BLATANT SIGNS OF FLIRTING. you've already said that he doesn't flirt with other people. he is obviously interested in you. if you like him (and it seems that you do), please flirt back. you can do it! he will be flattered. and who knows, maybe the two of you will end up having a very happy time together as more than friends...