Does she eat a little bit of that apple each day? :)
My point is, the tense is in the habitual aspect, so it suggests we are talking about apples generally rather than any specific apples.
Ultimately, it really doesn't matter from the standpoint of Hindi. If you want to make sure the sense of THE apples is conveyed, you can say /ve seb/, which may look like "those apples," but which will means "those apples [which are under discussion]"...i.e. the apples :)
Wait, how do I know whether it's "appleS" (plural), not "(the) apple" (singular)? "सेब" can be both singular and plural, can't it? And है agrees with "बहन", not "सेब", right? I would literally translate this as: "Your (feminine, refers to "sister") sister apple (OR apples?!) eat (feminine, refers to "sister") is".
I wonder why "the apple" isn't accepted... It's not wrong because in most languages that have no definite articles the word "the" is implied... I also have learned from my on and off trying to learn Hindi that if you need "an/a" not just implied then using "एक/ek" before it works. Like "तेरी बहन एक सेब खाती है ।/Tere bahan ek seb khatee hai." Since "A/An" in English denotes there being only one of the object!