My son is 9 now, me and sister in law were pregnant at the same time although I was due a couple of months before her. She never said it to my face but she was furious that I'd gotten pregnant before she had, it meant that I would give the in laws their first grandchild and she wouldn't. Anyway, as I was nearing my due date she used to say stuff like "If my DD was born now she'd be the eldest instead." (Bearing in mind that she was due two months after me!) On a few occasions she actually said she wished her DD would be born early and she would hint that her consultant said he needed to do an early Csection 
The babies ended up being born in their intended months, mine being first.
Anyway fast forward a few years and they moved away to the opposite end of the country and we hardly see them anymore. Recently, DN came up for a visit and her and my DS got talking about their birthdays and their ages and who was the oldest etc. They got into an argument because DN was insistent that her birthday was.... three months earlier than it actually is. DN came and asked us when her birthday is (she was at our house at the time) and we told her when it was, her real birthday, we showed her on the calendar. And she refused to believe us, she kept saying "So my mum's been lying to me my whole life about when my birthday is? My birthday is not 'X' but actually 'Y'??" We got her to ask MIL when her birthday was and MIL was like "Errm, it's , why?" And DN just kept saying "So my mum's lied to me this whole time?"
What the hell?! Who does that??