Hmmmmm.... wellll...
DS1 was 28 months when DS2 appeared on the scene. We had built him up to it, talking about how he was going to be a big brother, have to look after his little brother etc. When DS2 arrived he brought with him a present of a bike for DS1, so DS1 warmed to him immediately.
After 2 wks when DH wentback to work and DS1 was competing for my attention all the time the novelty wore off. Every time I fed DS2, 1 would appear and start driving toy cars over him, pulling his clothes or even hitting him on the head (that has only happened once, mind, as he was royally scolded). The HV suggested having a box of 'special' toys that DS1 could play with only when DS2 was being fed. That worked for a bit!
Now DS2 is 13 wks, DS1 loves him. He kisses and cuddles him all the time. He tries to share his toys with him, which has led to DS2 getting bumped on the head with hard toy cars a few times, so we've had to explain that 'while it's lovely to share, DS2 is too young for these. how about that nice soft toy instead?'.
The hardest thing to manage is his real interest in his baby brother, which means he pokes, prods and pulls him, i.e. 'look, he's got lovely little eyes' - poke.