Milestones don't matter.
Parents spend a lot of time telling their kid's things like "you're special" and "I love you just the way you are." So why is it that during the early stages of life, parents put their child in a box? During the first month they must be able to coo. During the second month, the child needs to smile, laugh, and hold their head steady... and so on and so forth.
Milestones do help parents make sure there child doesn't have a disability, which is a good thing, but be wary of putting too much stock in your child reaching goals at a certain time. If you're not careful, the dark side of milestones may rear it's ugly head.
Don't Enable Sibling Rivalry
When you start comparing milestones between siblings, it can only lead to bad things. Each child will progress at a different rate, and if the older child hears you saying things like "oooh, big girl, you learned ______ way before your sister," all it does is break down the older child.
My Kid is Better than Your Kid
It goes without saying that my kids are better than yours. My dad can also beat up your dad, and your momma's so fat she needs a boomerang to put her belt on. That said, whenever comparing your situation to someone else's, remember that more than likely someone else's child will be "better" than your kid at something. Someone's dad will be able to beat up your dad, and your momma is probably fatter than someone else's momma. We're here on this earth to help each other, building on each other's experiences, not to make sure our kid is "better" than everyone else.
Sit Back and Relax
Kick your feet up, relax, and enjoy the good times... after all, once your child reaches a milestone, there's no going back.