उसका/उसकी/उसके translates with the object, and not the subject. As well, when words like पास or बीच or सामने are used it always takes the उसके form. In fact, उस can be either a boy or a girl. We have no way of knowing which from this sentence.
So उसकी बहन है could be 'She has a sister' or 'He has a sister'.
Similarly उसका भाई है could be 'She has a brother' or 'He has a brother'.
And उसके दोस्त हैं could be 'She has friends' or 'He has friends'.
For a plural subject we use उनका/उनकी/उनके (Their brother, sister, friends, etc.).