Well, it may certainly be both of those things. If your parents showed some level of obvious favoritism or made it known that you were especially cherished, that certainly could spark some uncomfortable feelings with the others. It's probably also true (having 3 kids myself) that you as the youngest were raised with a little looser leash. Your parents may have been overprotective near the beginning (or had more energy then) and chose to back off somewhat. That is probably very natural, and it can be a challenge not to let it go too far.
However, no one has forced your siblings to behave in mean spirited ways. If they have chosen that path, that is their choice. Maybe not helped much by your parents (not intervening much, getting sucked in, etc), but still their choice to handle it in that way. When it becomes a "sick" family pattern, that can be so hard to break over years and years of bad habits.
Do you have much to do with them on a regular basis?