RE: sibling rivalry
my 3 sibs & I were wonderful to each other... at times also awful to each other. Ditto my hd & his sister. Ditto my dc.
From a parent's perspective, the most helpful thing for us was
1. affirm the child's *feelings* bc they are only natural & he has a right to his feelings (jealousy, anger, sadness, etc)
2. make it crystal clear that *acting* on those feelings in a way that is harmful to others is *not* acceptable (ie you can tell mom you're mad that your sister has your old crib, but you can't try to punch mom or pinch your sister when you're mad... or w/e works w your dc)
For my dc in the preschool years, just the affirmation that they're *allowed* to be upset with a tiny baby & even say so went a long way toward resolving those negative feelings & the clear line on where expression of feelings becomes wrong (ie hurting others) was just the security they craved when so much of our lives were changing (ie they seemed, in retrospect, to need the guidelines & know that the world hadn't turned upside down-- mom was still mom & had limits, etc)