Joyce's "The Sisters," from his short story collection, Dubliners, is extremely complex. For a full critical analysis of the story I suggest you click on the source below and read the enotes Study Guide on the story. I'll give you a few highlights.
The priest, the young boy's mentor, who dies, has suffered paralysis. This is emblamatic of numerous characters in the story collection and therefore indicative of the Irish, in Joyce's view, living in Dublin at the time he wrote the story, and by extension, of course, to people any time, any place.
In the past, the priest, as the boy remembers, entertained himself by "grilling" the boy about church procedures and traditions. He takes what should be uplifting for the boy, and makes it repressive. Sexual overtones are also present in the meetings between the priest and the boy as the boy remembers them, with the priest's tongue oddly hanging out.
The relationship is also not as it should be, as revealed in the boy's dream: the boy feels that the priest wants to confess something to him: a reversal of the roles as they are meant to be.
Finally, when he finds out that the priest has, indeed, died, the boy should feel mournful, but instead feels a sense of freedom. This reflects Joyce's view that the church stifles the people of Ireland, that it represses them.
Again, click on the source below for a full analysis.