Fights are very common among children. There could be disputes and disagreements regarding the dress the parents have brought, sharing stationery, sharing chocolates and ice cream, fights about the serial or movie one wants to watch etc. There could also be fights if any of the kids feel that the parent favours the other one.
Kids may also fight if the younger one feels that the elder exerts too much pressure; and the elder may feel that the younger brother/sister is always excused. There may also be a fight to exert each others power and position in the relationship. If you are an elder, you can follow some tips to avoid conflicts with the younger one.
Do not use your position of being an elder for your benefit. The fact that you are an elder does not entitle you to treat the younger one in any manner you like. Do not try to be the parent to the younger one. Just be what you are, i.e. the elder brother or sister.
Do not exert undue pressure over the younger one, to discipline them. Leave it to the parent. You can act like a friend and try to dissuade them from doing things that are not correct. Do not hit them or scold them at your will. This will create more hostility between both of you.
Do not sneak into their room or go through their things in their absence. The younger brother/sister will resent you going through their things, and your spying on them. Avoid reading their diaries and little secrets they write on it. Kids also need space and privacy.
If you have come to know some secret, do not threaten the younger one with it. The kid will find such behaviour cruel and mean, and will not be able to trust you.
Never boss over the younger one. Treat him/her as an equal, but help and guide them whenever necessary.