This is within the normal range, but you can prolong it or minimize it with how you handle situations.
For prevention, it may be mostly semantics, but at that stage I tried to never say I couldn't do something because of the baby. For example, instead of "I can't play with you right now because I'm nursing Baby" I say "I can play with you in 10 min. Can you tell me when 10 min is up?" Then, make sure you follow through.
I also never let the younger one get away with behavior just because he's younger. As an older sister, believe me when I say that younger siblings learn to milk that at a young age, long before parents realize it! Do not let your younger child get away with anything that your older child would be punished for - the rules should apply equally to both kids. To do otherwise is a recipe for instant rivalry.
Also, if there is a tussle, both kids always go to time out. Yes bigger sibling can't hit younger. But often younger does provoke it (sometimes unintentionally sometimes intentionally). I don't try to sort it out because there is no way for me to know who really started it. They both go to time out every time. Sometimes they bond over being mad at me, but I'd rather they be mad at me than at each other.