Ex bf, me and sister ...and why he is acting the way is?

here is the short version of the story:i met this guy 22 months ago.we attended the same comm. college as well as the same university for the past 2 semesters.this summer with school-summer cles-he was beginning to drift and became more distant..he started acting like a jerk made this joke/i am kidding comment that i looked like i put on weight in 3 weeks,that i look pregnant,canceled plans for Pirates 3 the 1st day he said he would go with me-ok i give him props for coming from work and driving to see me-making me cry..etc..then i went to the movies with a good friend that i know longer than my now ex-3 years- and we ended up kissing which by my standards is called cheating..nothing more has occured between us except that me and that friend have not spoken since.i waited almost 1 week and a half and told my ex about the kissing thing.he said that we are done and in 4 days he somehow moved on. his sister and i are really close yet he dislikes it and wants nothing to do with me