All these declensions are a little overwhelming...
I can just about work out why a particular ending is used, once I see it, IF I study the declension table, but still struggle to choose the right one.
Can anyone offer some advice, is it worth sitting down with a huge declension table and memorising all these endings, or does this soak in after a while and you just pick it up naturally?
If you're still struggling with this, and based on my experience you probably are, I would suggest: every time you encounter a new word, plug it in here http://starling.rinet.ru/cgi-bin/morphque.cgi?flags=endnnnnp or find it in Wiktionary or something and read the declension table, and eventually the patterns will start to stick.
Well, this depends on the age of girls. It's impossible to tell without a context.
If the brother is still in the middle school, and thinks about his female classmates, then «о де́вочках» works perfectly well.
Of course, «о де́вушках» should also be accepted. We don't have any context in Duolingo, so both work well.