But мы is we. So literally it's like "we and (my) sister..." You have to understand to a native English speaker it sounds bizarre when it is first encountered. But if such is a grammatically correct and common Russian expression, we just have to grin and bear it (i.e. learn by heart)...
You are probably using the mobile app, just like me. If you've never visited Duolingo's website before, you'll be surprised to know that every topic has a "Tips" section that introduces and explains the ideas and words used. This is one of the things discussed there.
It's something that really annoys me to be missing from the app, but having duolingo bookmarked on your browser, and remembering to visit the site before each new topic works.
Some of the other languages have added the tips section to the mobile app, so it's not impossible. I've seen it in Spanish and French so far. Russian is less common, so less priority maybe, but hopefully they will bring this feature to all the languages on Duolingo eventually.
In fact, if любить is adressed to someone, like тебя or его, it means "to love", but if it is adressed to an inanimated object, an action, etc... it means "to like". Nevertheless, in english it is correct to say "i love reading", so it should be accepted (i got it wrong too, from the same "mistake"
Just keep in mind you cannot learn a language with a word for word translation of English. Some phrases have to be learnt by heart that's all. As a French native speaker, I think the Russian is often more logical than the English. The main mistakes I made are not in Russian but in English :)
Because "со" is typically only used when the following word begins with consecutive consonants. Having the following word just start with "c" but without consecutive consonants, like "с сыром" means the "c" sound is lengthened when spoken, so it sounds like "сссыром," a slightly elongated "s" sound. E.g., when to use "co" -- "со снегом."