Most Helpful Girl
Just because two people Breakup doesn't mean they can't and don't Makeup and that it is Goodbye, my love forever. And in your case, when you saw one another's face, there was Definitely Jealousy inside when he saw you With... The guy.
I would imagine there may have been a flame or so left in the pit of your own tummy here, dear, when 'I just walked away' and said not Even a "Hey." And when your sis happened to Notice His own reaction, this is telling me that you both still have a cold coal or two left over but no one has bothered to shout one another up 'For almost 4 months' because you both have been trying to Move On... With dates and mates.
Good luck. xx0