Translation:Her brother bought tickets from the ticket booth.
It would be "Ο αδερφός τής αγόρασε εισιτήρια". Please see my comment in this discussion https://www.duolingo.com/comment/17783932. Check also this one https://www.duolingo.com/comment/20059064 and search for accented pronouns "τού", "τής", "μάς" etc. Many more discussions will be listed.
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