The common lovebird species (Peach-faced, Masked, Fischer's) are not sexually dimorphic, and thus you can't tell "just by looking" if they're male or female. You'll hear some people say you can observe some characteristics that tend to indicate "most likely male" or "most likely female" but none are 100%. To know for sure, you'd have to have it DNA sexed.
You are half-correct about the identity of the lovebird -- it's actually a Peach-faced / Masked hybrid. They are sterile, so no need to worry about finding an opposite-sex mate for breeding -- it won't happen even if you do. And I wouldn't even worry about getting a mate if the bird is friendly, anyway. People think lovebirds need to be kept in pairs because they recount the origin of the name "lovebird" -- pairs will often sit so close together it was thought that they'd "die if kept alone." That's not true. Single lovebirds can make great pets if they are people-friendly. If not, certainly they can enjoy interaction with a bird-buddy, but it doesn't have to be of the opposite sex, and it doesn't have to be in the same cage, and it doesn't have to be another lovebird.
Let your sister give the bird some time to get used to its new home. Keep the cage in an area with lots of human activity. Tell her to offer favorite treats by hand so the bird gets to know her better. If after a month or so it becomes apparent that this bird wants nothing to do with people, THEN consider a possible bird buddy. But not as a cage-mate -- at least, not right away. More of a cage-neighbor. If, after having both birds for a while, your sister thinks they get along well enough to share a cage, then a NEW and larger cage should be bought -- one which neither has claimed as its own, and thus will be "neutral territory." And keep in mind that should she decide on keeping two in a cage together, that will mostly nix the possibility of either being a friendly hand-tame pet.
