What's wrong with "Min søster var i flåden når hun var yngre."?
You use når in present tense, but in past tense you use da instead. Both words mean the same thing though, just the usage depends on the used tense.
"navy" also translates to "søværnet"
What is the official name for "The Navy" in Danish? Is it "flåden" or "marinen"?
It's actually "Søværnet". Both that and "marinen" should be put up as correct answers.