What should i do about this?

Okay so I've been dating a guy for a month now and today i was at his house and his step brother was there and i don't know what happened but we kissed for a 2 minuets and i don't know what to do i know i should tell my boyfriend but he's a jealous guy if he sees my hugging a guy he will get mad and we try to pretend it never happened but his brother likes me and when my boyfriend is not looking he is always flirting with me or touching my leg what should i do.... im 15 there both 16 and there really close i don't want to break them apart.