My SIL wants my 11month olds birth certificate????

[1sttimerdue260816]

My SIL has said she wants my 11 month olds birth certificate so she can get something for her 1st birthday. I find this odd. She said she wants it to be a surprise. I don't want to be a spoil sport, but she's a bit odd (and she really doesn't like me or my DH - jealousy issues from childhood i think) and I'm worried she might do something weird.
What might she need her birth certificate for? she said she will only need it for a day

I opened a Halifax junior isa for my DD recently and didn't need a birth certificate, just her name and DOB.

Only slightly harder to get info for someone else to get hold of was NI numbers for myself and my DP.

Maybe a different sort of savings acc you do need birth certificate...but not for ISA