I'm no expert - crikey you only have to look at what my two boys get up to, but it seems to me that its probably perfenctly normal and just the adjustment from being an only child, to being one of two (one of whom is 'just a baby' and therefore not much of a threat because they are 'something different') to one of two children who are both little individual people and who both have a right to Mum and Dad's time/attention/love.
My boys were exactly the same, ds1 tells me quite openly that he has always been jealous of ds2, but that he also loves him more than anything 'on the planet' oh 'except dd' apparently. (Sometimes ASD type behaviour can have its uses, as ds1 can tell me things like that without it even entering his head that it might hurt his brother's feelings if he says he is jealous of him.)
He used to pretend to cuddle ds2 whilst reaching around the back of his head and pinching him on the neck - we have photographic evidence of this particular trick! His other trick was to tell a very compliant ds2 to do something that he wanted done, but knew was naughty or against the rules, then revel in ds2 getting told off for it by adding lots of 'Yeahs' to the end of my sentences and saying things like "Oh, Mummy that's so naughty what ds2 just did isn't it? He knows he shouldn't do that doesn't he? I wouldn't do that would I, because its naughty." etc etc. (He still does that by the way.) Ah, brotherly love eh?
Oh, sorry! YES! Absolutely, its a phase. They will grow out of it in no time at all and become bosom buddies for the rest of their lives.
Seriously though, they do love each other but have conflicting feelings, as they also have a raft of negative feelings towards their siblings as well. My boys hate to be apart, insist on getting a treat for the other one even if they are getting a treat because they have been to a hospital appointment or something and are a completely united front when it comes to nasty mean parents like dh and I. I am in no doubt that they will be firm friends for life, regardless of where life leads them both.