To spend more money on my eldest daughters birthday then my youngest

[Differentstarts]

I have 2 daughters 6 and 3 their birthdays are both next month. My 6 year old doesn't have a dad and only has me. My 3 year old does plus all these other relatives from his side. There's no relatives my side.
I like to treat them both equally and spend the same amounts on them to keep it fair. But doing this my eldest always ends up with so much less as my youngest has this whole other family buying for her. Aibu spending a lot more on her to even it out.
Yabu: you should spend the same on both
Yanbu: You should spend more on the eldest to even it out.

What station = way of looking at it

Yanbu to buy more presents from you for your elder daughter (provided that they are things you think she needs or wants). I probably wouldn't try to match the number your younger daughter gets as a) if she does get 30 presents that seems like more than I would want coming into the house and b) your younger daughter is only 3 so the number of presents off extended relatives is likely to decrease over time anyway.

And you wouldn't be unreasonable to try to mitigate things in other ways.

You could decide that as a household you buy things when you need them, which over the course of the year would mean spending more on the elder daughter.

You can seek to support individual relationships for dd1 with other trusted adults that you know. For example a neighbour teaching dd1 how to knit or one of your friends showing a particular interest in one of dd1's hobbies.
You can set the rules in your house when they are teenagers so that a part time job becomes a must have for life skills reasons.
You can also decide on rules about passing things on. Each daughter's things are hers until she decides she no longer wants them and then can be sold and the money added to a savings account. This may mean you selling things to yourself on behalf of each daughter, but would allow you to build some savings for each of them and stop the younger daughter "taking" the elders things. I know you haven't mentioned this happening but lots of posters have talked about the younger daughter automatically getting the olders things and I've noticed elsewhere a pressure on older children to pass toys on as soon as the younger child reaches the lower age range limit even if the older child hasn't reached the upper suggested age limit.