Annoyed!

My fiancé of almost 6 years is currently doing my head in. Probably natural as we had a baby 2 months ago. Anyway he's 36yrs old (I'm 35) and something he told me yesterday that's annoyed me. My 18yr old niece whom I hardly ever see now always messages him...not me. She told him last night that she's pregnant. I immediately got offended because that's something she should be telling me. She asked him to keep it a secret. Yes he came and told me but isn't that a bit weird? Even though I haven't seen her much I was always around when she was growing up, getting get toys and things for birthdays, christmas etc.