"Your sister eats apples."
Translation:तेरी बहन सेब खाती है।
I think it is both correct. Just more formal.
See also an answer to https://www.quora.com/What-do-tu-tera-tum-teri-tumhara-and-AAP-mean-in-Hindi on Quora:
Tu, tum, aap- When translated to English, all three correspond to 'You'. In Hindi, the usage involves increasing degree of formality and respect. So Tu is used for very close friends/junior and totally informal while Aap is used to convey the highest degree of respect and formality, used for seniors/parents etc.
Tera, Teri, Tumhara are again similar, equivalent to your. Tera and teri are more informal, Tumhara and tumhari are less informal. Aapka and aapki are the formal words. Tera/Tumhara/Apka is used when object is masculine. Eg. Tera ghar, tumhara ghar, aapka ghar (Your house). Teri/Tumhari/Apki are used with feminine objects. Eg teri kitaab, tumhari kitab, aapki kitaab (Your book).
Try looking at it this way - the verb will correspond to the subject of the sentence.
In the sentence, "तेरी बहन सेब खाती है।", the subject is 'तेरी बहन' (your sister); thus, the verb 'खाती' (eats) corresponds to that.
Similarly, in the sentence, "हम केले खाते है।", the subject is 'हम' (we); thus, the verb 'खाते' (eat) corresponds to that.