Okay, maybe that's not so easy. But Don Carlo has what can only be called rather tangled relationships among its principal characters. Here's a handy guide to at least some of what's going on. There's some ambiguity in the relationship between Rodrigo and the King, and Eboli and Elisabetta's relationship, initially warm, undergoes, ahem, a radical shift after Eboli's double, or maybe I mean triple, betrayal of the Queen. I will happily accept clarifying suggests about this graphic; just add them to the comments.
I would definitely suggest a close reading of the libretto. I happen to have the tri-lingual ENO libretto, which appears to contain every word everyone ever wrote to be sung in this opera. I alternated between reading the late Andrew Porter's singing translation and the original French.