ё and е are completly different letters. Сестры (with a stress on ы) is a genetive case of сестра, and сёстры (ё is stressed) is a nominative plural form of сестра. Word сестра with stressed е doesn't exist at all :) But notice that native speakers sometimes replace ё with е, but, in my opinion, it is incorrect.
I am certainly one of the people, who think ё is an atavism and must die. 99.99% of the time it's clear what the word is from the context even when using е in place of ё is ambiguous. (And when it's not crystal clear, it makes for some funny ambiguities.)
I don't think I've ever used ё in any written sentence since elementary school some 30-some years ago and I don't think I have ever been misunderstood because of that.
I'll admit that it's probably easier to learn Russian with ё because it simplifies a lot of those ambiguities that native speakers sort out without thinking about them.
This is why I said 'in my opinion'. I don't like it, but, certainly, е often replaces ё.
Yeah, i could easily read that fasr in English beause im a native speaker But im a fkreiner in Russian. I cluld do fine with её, but if jm learning a new word i appreciate knowing where the accent goes. Сестры/сёстры was not automatic for me til i read the explanation above.