I think people have been fairly unsympathetic, its hard when you feel your child is the worst behaved in a group - and especially worrying if you see family members with similar 'traits'.
Your son is still a very little boy though, so you have time to help him if you feel his behaviour needs changing. Children don't really 'get' empathy until they're 6 or so, so being selfish is fairly developmentally normal.
Also, the need to be first etc is normal - my ds is 6.5 now and until v recently counters would be flung/strops had if he didn't win at games, its hard, but we found talking about it beforehand would help - e.g. 'we will play a game if afterwards you can say well done to whoever wins and we'll say well done to you if you win.' It does pay off in the end. We also played (simplified) versions of games like Pop to the Shops where we could model good game playing with lots of little success/failures within the game 'oh well done you got a 6, you're nearly there, wow look you bought another food' 'oh no I only got a 1, I'm so disappointed, oh well, I hope I get at least a 3 next time! Its hard work, but it does pay off.
Also, praise anything and everything that he does that is working towards the behaviour you want. Pick a focus - e.g. sharing - ask him to give something that he doesn't much care for to his sister and then praise him to the hilt when he does 'ah you are such a kind big brother, thank you for sharing (big hug/kiss, whatever). dd say thank you to ds.' Keep going and he'll want to keep being kind etc to get the praise & attention and also you'll be used to noticing the positive things he does so you'll feel more kindly towards him as well, which'll make your relationship with him better.
Think of anything thats something he could deal with and instigate a sticker chart - 10 stickers and he gets a treat. Also, screen time can negatively impact on behaviour, so limit that if possible. Lastly, I'm sure you don't, but please don't compare him to dd, or let anyone else do it as it'll make his behaviour and relationship with her worse.
Good luck.