I hate to sound like a broken record, but it again feels like I have to say this every time a subject like this is brought up until people start to remember it themselves…
FIFA does not require a region to be independent to join.
FIFA Statutes, on page 11:
“An Association in a region which has not yet gained independence may, with the authorisation of the Association in the country on which it is dependent, also apply for admission to FIFA.”
That means that if the KNVB (the Dutch FA) give their authorization, Sint Maarten can become a FIFA member.
I also hate to sound like a broken record, but we all know this already. And we've had many discussions on it before. FIFA does not require a region to be independent, but the case of Zanzibar (for example) is contradictory. They had the backing of the Tanzania FA, yet FIFA turned them down as they were not a recognised nation, despite the fact they had a separate league and a separate functional national team playing in the CECAFA Cup for over 60 years, and they were also a separate member of CAF.
And as Pieter says, confederational membership has to be sought first, and then only after a certain period as a member of a confederation can a member join FIFA. As certain confederations have contradictory admission rules (CAF and OFC, for example, somehow state that a country needs to be a FIFA member before it can become a member of their confederation, despite the fact that the opposite is required - thus making it impossible for any new team to join, never mind non-independent territories).
It's not as simple as a parent body giving consent and then everything is all fine. The parent body of Gibraltar (which was technically England), gave their consent for Gibraltar to join. Yet, we all know what happened there.
Also, technically speaking, Sint Maarten had a separate FA already, as they were already CONCACAF members. So there was nothing to stop them applying to FIFA before.