The sad thing is, I should have known nearly all these things. Wasn't I a kid? A sibling? Did I not have parents? Somehow we have to learn it all over again.
Your children will always be totally different, even If they are the same sex. My children (one of each) like different books, TV shows, food, school subjects, and so on. One dances, one plays baseball. Its no doubt a conspiracy to keep us from being bored.
They will fight. No matter what you do. They will fight over the stupidest stuff. They fight over who has the remote on a show they are both watching. They will however defend their sib to the death against anyone else.
While we are on that....sibling rivalry is. My kids compete over whose picture hangs above the other in the living room. The eldest says the younger may receive no privilege or gift at a younger age than his. Last year both got 4.0 one grading period. They argued over whose was better, the one who had all As or the one with a B but getting extra credit for AP classes. Recently I advised a friend to give her daughter another middle name, to match her brother. You will hear about it forever if you don't I warned her.
They don't break as easily as you think they will. I am speaking both emotionally and physically. They fall, tumble and collide and come up smiling. They take emotional blows that would lay us up for weeks and march right back again.
They are great company. I thought as a kid my parents took us to things because it was their duty. Not till I had kids of my own did I learn how much pleasure it was to visit a favorite place with them, or watch a favorite film through their eyes.Right now I am travelling alone and while I am enjoying the quiet, I keep thinking I should be sharing this with them.
When you bring that newborn home, never mind what the doctor said about breast vs bottle or schedules; if they are screaming and the diaper is dry they are hungry feed them. If they refuse the breast and are still screaming they are hungry and frustrated. Feed them.
They will when you least expect it surprise you with their courage, their compassion, and their maturity.
It really does hurt you more than it does them.
You will worry. Always.