Translation:I am home, big sister.
So from what I understand, even with your own family, if someone is older than you, you use the same title to address them as the one you use when you're talking about other people's family members. They're older than you, so you show them respect. So "お母さん、お父さん、お姉さん、お兄さん". Of course in real life that often gets shortened to "kaasan", "tousan", etc etc, but that's a really informal colloquial sort of thing, and so isn't going to appear in Duolingo.
If you're addressing your own family members who are younger than you, you generally address them by name.
はは、ちち、あね、あに are not informal forms but rather sort of like... humble forms. You use them to refer to your own family when talking about them to other people
ただ means "just" and いま is "now", so a literal translation would be "Just now!" which is obviously not a complete sentence. This is a set phrase and, like many set phrases, it is an abbreviated version of a longer, unspoken sentiment. In this case, it means something like "(I've came home) just now." or "Just now (I returned safely)". It gets translated as "I'm back" or "I'm home" quite frequently, because that matches how it is used pretty well.