I can't hear "ё" there. You are right about stressing "ё" in general and about the stress in singular "сестра".
The pronunciation is really confusing, but you have to keep in mind that it's a computer-generated voice. There are some services like [https://forvo.com/languages/ru/] where you can listen how locals pronounce the words. That's a better way to get a good pronounce.
Thanks, but a lot of us non-native speakers (at least gph17 and I) are hearing a "ё" in "сестры" (9/20/2018), which is apparently wrong. DL pronunciation editors, can you check this?? If it's a computer voice, and it sounds that much like ё to us, either it's entered wrong in the computer voice, or we need to learn a quirk in phonology (or maybe both phonemes are allowed here - what do I know?).