I would start training Caleb right now to enunciate his sister's name correctly. Everytime he lisps "IS-Ball", just bare your teeth in a frightening manner and SCREAM at him: "HER NAME IS IS-A-BELLE! IS-A-BELLE! GET IT RIGHT!" until he cries.
See, I'm one of those people who doesn't have kids of her own, but has LOADS of free advice for folks who do! Ever notice how many of those people there are around? ;)