H.
I'm the oldest of three children and have two children of my own that are very close in age.. just a lil over a year apart.
What my husband and I did to combat "sibling rivalry" is these two tricks that worked best.
1) Take the object they are fighting over away and put it up for the rest of the day, when they end up with nothing to play with they will start sharing.
2) for the older kids buy some "sockem bopems" the inflatable boxing gloves and have a "boxing" match who ever wins *usually the first one to land a hit* gets to play with the toy first and then after snack , lunch *insert specific time* the other gets to play with it...
What ever you do don't say " well he/she is just a baby and doesn't know any better " or " he/ she is just a baby and your a big boy/girl so you should let the baby play with it. That will only cause bigger issues than a few fights over toys.
The other suggestions i've read are also very good. ALSO, one BIG life saver for me was "MOMMY TIME" I set aside a time every night where I get to do what I want and my Hubby takes over domestic duties. Usually I light a bunch of candles in the bathroom, run a bubble bath, grab a good book and some earplugs, and i lock the bathroom door, It helps me to rinse off the stress of the day before bed.
My kids are a bit older than yours so my methods may not work right now, but may in the future.
I was where you are just a few years ago, as they get older, the boy won't want to play with the girls things, and having had a sister 3yrs younger than me, buy all the girl toys in duplicate..it will save you alot of grief in the future.