16ccunning -- I don't think that they expect us to know every Spanish word. I think that they probably expect us to supplement this program with other online and offline resources. As you can see relying solely on duolingo to learn Spanish does not suffice. I use a book called "501 Spanish Verbs" to study verb conjugations. Also "Barron's Spanish - English dictionary. And a good "online" website is "www.spanishdict.com." Hope that this helps.
If you wanted to say "My sisterS do not eat cheese" then you would use "mis". "Mis" is the plural version of "mi", so if you only had 1 sister then you would say "mi hermana". Whereas if you had 2 or more sisters, you would say "mis hermanas". (more examples: my dog = mi perro, my dogS = miS perroS. my cat = mi gato, my catS = miS gatoS.) Does that help?
If you hover over the words you don't know it gives a translation. I find this actually really effective- it helps me learn words by context and root the same way I learn words I don't know in English when reading a book (first try to figure it out from context, then see if I know a word with a similar root that helps me figure out what this word might mean, then finally look up a definition/translation of the word. This is something most kids learn to do when they become fluent readers in their native languages.)