Help!!!

I'm pretty sure my sisters boyfriend tried to come on to me... The first time was no to long after they had my niece he asked if I could teach him to eat the box and said to keep it between me and him. I informed him that there was no way I could do that and It's inappropriate to talk about this. And now he hits me up on Facebook asking to chill and could I pick him up. I said no and asked why is he acting inappropriately. He was really trying really hard. But I denied every time. So my sister asked could they stay with me for a couple weeks until their place was ready. I said yea but wondering if I should tell her... This is the happiest I have ever seen her and she even got his name tattooed on her arm. I don't want to mess up our relationship because this also is the closest we have ever been. What do u guys think. Am I as guilty as he is by not telling. Because I think I'm considering her feelings in the matter.