The poem contains numerous allusions involving religion, rock 'n' roll, or both.
For example, the poem's two epigraphs refer to an Elvis song and a Madonna quote. (As in the rock star Madonna, not the Virgin Mary.) The first quotes the lyrics of a song Elvis recorded in 1960 called "Are You Lonesome Tonight?" The second paraphrases what Madonna reportedly said upon meeting pop artist k. d. lang, whose style and talent reminded her of Elvis. (The actual reported quote was, "Elvis is alive—and she's beautiful!")
These allusions frame "Sister Presley's" monologue as a testimonial from a female version of Elvis—his imagined, very similar twin. The lyrics may also suggest that Sister Presley is addressing fans who "miss," or feel "lonesome" for, her brother—their now-dead hero.
And all of this comes before the poem has properly begun! The allusions keep raining down from there. Lines 4-5 ("pray for the immortal soul / of rock 'n' roll") pun on "soul" music—a genre related to rock—while recalling other references, such as these lyrics from Don McLean's 1971 rock song "American Pie":
Do you believe in rock 'n' roll?
Can music save your mortal soul?
"American Pie," too, mixes rock references with religious language, and one of its verses is widely believed to refer to Elvis (by his nickname, "The King").
The Latin phrase in line 13, "Pascha nostrum immolatus est," comes from a traditional Easter hymn, and translates to "Our Lamb (i.e., Christ) is sacrificed." This choice of quotes may suggest that Elvis, the speaker, or both are martyrs of a kind. After all, Elvis died young—overdosing on drugs under the intense pressures of fame—and Sister Presley has withdrawn from the world's heartbreak to join a convent.
The last three stanzas pile on several more Elvis references, alluding to his song "Blue Suede Shoes" (line 20), his "Graceland" mansion (lines 21-23), his song "Lawdy Miss Clawdy" (line 26), and his song "Heartbreak Hotel" (lines 28-30). Clearly, Sister Presley loves her brother's music and finds both its joy and heartbreak relevant to her own life.
Finally, the poem's overall concept may be a winking reference to the Gospel of Matthew (16:18), in which Jesus founds the Christian church "upon a rock." The speaker's belief system certainly incorporates Christianity and rock music alike.