Why do some entries not have complete declensions?

I am a beginner learning Swedish. I love the Tyda.se format, but sometimes I find entries that do not show the complete declension; look for example at the entry for 'syster'. 

syster

systern (-) (-)

What are those (-) supposed to mean? Would it take so much effort to just type out 'systrar, systrarna'?
I mentioned this on the Duolingo Swedish forum and they recommended me to try Wiktionary. Though I like Tyda.se's style better, I am switching to Wiktionary because it doesn't omit information.