"My sister never drinks."

Translation: Η αδελφή μου ποτέ δεν πίνει.

[ShaoxuanLi]

are πότε different from ποτέ; πότε is when (asking for a time) and ποτέ is never?

[troll1995]

Yes, exactly.

[Mark_B420]

Is αδερφή just a spelling variation?

[mizinamo]

No, it also corresponds to a pronunciation variation.

αδελφή is the "classical" form which was resurrected; αδερφή is what the word turned into by centuries of sound change, at least in some dialects.

Nowadays, both forms coexist.

[Dmatafonov]

Is it the same for αδελφος και αδερφος?

[mizinamo]

Yes, exactly.

[Celso491350]

Η αδερφή μου δεν πίνει ποτέ. ή
Η αδελφή μου ποτέ δεν πίνει.
υπάρχει κανόνας;

[jaye16]

Please read the comment above yours.

"They mean the same thing."

[creyesm1992]

Why ποτέ (never) is followed by δεν? I think ποτέ alone suffices

[mizinamo]

ποτέ (never)

It may make more sense if you think of ποτέ as "ever".

Τον έχεις ποτέ δει; "Have you ever seen him?"

So it needs a δεν to make "not + ever = never". Ποτέ δεν τον έχω δει "I have not ever seen him = I have never seen him".

Similarly with some other words such as κανείς -- which by itself means "someone" and needs a negative to make "anyone" out of it.

Κανείς έφαγε το ψωμί μου! "Someone ate my bread!"

Κανείς δεν τρώει το ψωμί του. "Nobody (= not anybody) is eating their bread."

[creyesm1992]

Thanks mizinamo :-)