Greek schoolgirl immigrating

[NatCam]

Hi,
My niece wants to come and stay here, in the UK. She's Greek, will be 17 in November, goes to school.
My question is: what kind of medical insurance does she need in order to enrol in school and stay?
Another question: can her mother and grandmother join her later this year? Is it possible to immigrate after 31.12.2020? Mother and grandmother are non-EU, mother is a permanent resident of Greece (20+ years).
Father is Greek. Father will be staying behind as the parents (of the girl) are separated.
Thanks!
We really, really need some help.

[Zerubbabel]

Hello

She can't sponsor non-EEA relatives while she is under 18. She needs to be 18+. As she is 16-ish now, we are talking about year 2021.

By then, in theory, the EU Settlement Scheme would've been scrapped and new entrants would have to follow the UK immigrate law in place by then.

[Obie]

That statement is not correct, the parent of the 17 years old child can easily qualify under the rule for pre-settled status on the basis of Chen, so to say a child under 18 cannot sponsor, does not seem quite right to me.

[NatCam]

@Zerubabbel and @Obie many thanks to both of you.
What kind of insurance will she need? Is EHIC enough?
We lived in Greece for more than 10 years, My husband is British.
Sister married to a Greek citizen, currently separated.
The girl wants to move to the UK, she's been bullied at school, very unhappy.
My sister cannot leave her on her own, understandably.
Grandmother has lived in Greece for more than 15 years, is totally dependant on us. We can't leave her behind - she's blind.
What is the deadline - December 2020 or June 2021?
Many thanks.