Permanent Residency / Green Card Question

Hi all, hoping someone with a bit more understanding can help me!

I'm (tentatively) in the early stages of starting to look at emigrating to the US and am poking around the residency / green card requirements. I'm British, aged 25 and unmarried, and the daughter of a US permanent resident. My father has been living out there since 2005 and holds a green card, he is also married to a US Citizen - with this in mind I'm looking at the section on USCIS about Green Cards for a Family Member of a Permanent Resident and trying to work out if I would hypothetically qualify.

Does anyone with a bit more knowledge know what criteria I would need to meet? From the website I seem to understand that I would probably qualify and therefore be put in a "family preference category" meaning that I would get a visa at some point when numbers allow... However I keep thinking 'that sounds FAR too easy' and am convinced I'm missing a catch somewhere.

I guess my question is this: Have I got the wrong end of the stick, or am I missing something fairly obvious here? Are there any additional criteria I would need to meet?

Thanks in advance - I'm only in the early stages of considering this as a possibility, would really appreciate any clarification!

Hi - how old were you when your father married his US citizen wife?

As a green card holder (permanent resident),can petition for certain family members to immigrate to the United States as permanent residents. Like Spouse (husband or wife)
Unmarried children under 21
Unmarried son or daughter of any age