Maybe its just the Android version, but doesn't it seem odd that duolingo doesn't bother to teach you any of the concepts like noun endings outsidr of occasional questions like these? I had no idea that there were even different forms of words like сёстры until this question came out of nowhere
Not "soy" but "syo". ё sounds like the "yo" in "York" (the city).
Ы ы, has no English (or Spanish) equivalent at all, though you might think of it somewhat as an intermediate sound somewhere in-between the "u" in put and the "i" in kit. A very first approximation is "a i preceded by a very short u" but that is only a very very rough approximation.
I've found videos like this helpful, as well as listening to the sound being pronounced in various different places.
I wonder why this was downvoted. In the Android version of Duolingo, there aren't any notes with grammar information and people can easily get lost.
If a feminine noun ends with "a" and is not preceded by г, к, х, ж, ш, щ, ч, replace it with ы. Otherwise, replace the ending letter (a, я, ь) with и.