It's common, and benefit-of-the-doubt goes to firstborn status (like ioawjes said). However, do watch out - my grandparents are incredibly sexist to the point where my dad had to tell them they weren't allowed to get presents for my brother and not me or my sisters. My uncle is actively trying to fight the ingrained sexism (he's scatterbrained, and called feeling guilty when he realized he'd sent my brother a graduation check, but not me. I hadn't told family since I graduated off-cycle, I kinda feel bad now for not giving him the chance.)
Anyway, you didn't ask for this, but my dad was always great at fairness, including things like:
1. Using rules to dictate gifts: i.e. our allowance was tied to our age, there was a specific cap on birthday gifts for friends, etc.
1a. Use rules for activities - we could pick one weekly lesson (music, art, sport, whatever), so we had to prioritize, but nobody was given extra just because they wanted it.
2. Being aware of child psychology, and using it. My brother is 3 years older than me, so he had extra rules like "you can get more expensive toys, but you have to get the same QUANTITY of toys as your sister" since toddlers don't care how much something cost, just how much of it there is. So he got extra (since higher allowance due to age) but had to practice empathy and fairness.
So, yeah, option 1.