I'd strongly advise against getting a tiel (or any pet bird) as a birthday gift especially if you cannot afford it in the first place / do not have a stable income
Birds cost a lot to be cared for, they require intensive research/time/preparation/$$$.
Did you know that regular vets will not accept birds? You need to find an exotic or avian vet, that's right... exotic.
Did you know that you cannot use things like teflon pans for cooking in your home or it will kill the bird? and so much more...
I understand that you mean well and you want to give your sister the best gift possible.
But please think about both your family and the well-being of the bird.
Without your own money, what will happen if the bird gets ill? Your sister is very young, I do not think you want her to go through the stress of not being able to help a sick pet or even losing one.
So please understand.... do not get a pet with the circumstances you're in.
Instead, I suggest:
1. Discussing with her (realistically!) the future of getting a pet cockatiel
- Start researching together with her the realities and preparations of owning a pet bird
- Start considering rescues/adoption together with her
- Teach her that a pet is a forever commitment (most larger pet birds live long!)
- Research for avian vets near you
Please.... I hope you will think it through again. I only have good intentions for both of you, it is just not the right time ><
(had to comment because i'm also someone who loves birds too much)