I am a god mother to 7... yes 7 boys! I swear they don't trust me with their girls

lol
in all seriousness, being a godparent is a wonderful role, i don't expect anyone to hand any of their kids over to be should, god forbid, anything happen to them, with the exception of both my nephews - i'd fight their father to the end of the earth rather than let him have them! It's lovely to know that someone trusts you enough to be such a special part in their child's life.
B - Honestly for me with my infertility being a god parent was a wonderful ask, like i say i have 7! To know these children will be a part of my life even if i never had my own, really was an honour. I wouldn't count your sister out just because your eldest sister already asked her.
Over here (UK) i believe the tradition is if it's a boy = 1 x godmother and 2 x godfathers, if it's a girl 2 x godmothers and 1 x godfather. However you can have have as many as you like and often people pick couples so have 2 of each.
LTTTC #1 for 13 years
(16.10.12) Diagnosis - 'Unexplained Infertility' 
23rd Sep '13 - BFP - Became an angel October 1st
26th May 2014 - BFP - Please be a sticky bean!!!
7th July 2014 - ultrasound #1 @ approx 9+1 - TWINS! 2 beating hearts

9+3

8+6
14 Jan '15 - My Babies were born...Samuel Stuart 6lb 5oz & Oliver Michael 5lb 2oz