Stina as a name is okay. It neither stuns me with awesomeness nor bores me with ordinariness. It just kind of is.
Stina as a nickname walks the fine line between new-cool and lame-try-too-hard. Like
Topher for
Christopher.
Of your options, I pick
Augustina nn
Stina or
Justina nn
Stina.
Three boys nappin', one mama namenerdin'
To peruse my top 100ish names:
http://www.behindthename.com/pnl/82589
"My Inner Birthing Mother Paddles a Canoe" and other non-name musings
found on my blog:
http://jodifriesen.blogspot.ca/2014/07/paddle-power.html