Am I being cold hearted?

My husband (married for 6 years) and I found out that we were expecting after 2 years of trying and my mother in law told me we shouldn't tell anyone right now. I was thinking it was because it was early but during a conversation last night she said that it was going to upset my sister in law who's been trying for a few months. After years of praying for a baby I can't bring myself not to be excited. My first thought was "no one considered my feelings when they were announcing their pregnancies". Maybe I'm being immature. Maybe not. Please help :-/