P.M.
Okay...maybe this is a little hokey...and it's a little thing...but when my kids (who are now 11 and 13) were very young....each night at bedtime, when I prayed with them....I would end the prayer with "and thank you for my best friend __________ (brother or sister's name)". It was a little way of ingraining in their minds how important their sibling was to them.
When they got a little older, if they got mad at each other, I would sit them down and make them say nice things about each other....one for each year of age. I.E....the 8 year old would have to find 8 nice things about his sister and she would have to find 5 nice things to say about her brother. Usually, they ended up laughing. Sometimes, I would make them put it in writing. I still have a few of the lists...bad handwriting and spelling mistakes and they are treasures.
My mom set my brother and I up to fight with each other and I swore it would never happen to my kids. And ya know...my kids (who are 2 years and 10 months apart) have each other's backs now. They support each other and love each other. They are not allowed to make fun of each other or pick on each other and they just accept that that's the way it is. I believe they have truly learned to love each other. They see how some of the neighbors treat their siblings and they comment on how wrong it is. Fights are rare in our house. I think there are a lot of little things you can do to foster good relationships as time goes on. Yours are a bit young now because it would be all one-sided, but it's never too early to think about it.