i’m a bit late to this, but i’m fifteen and have a seven year old sister (which is almost the same gap). though i will admit i also have 12 and 9 year old brothers in between us. 
my younger brother and sister are the siblings i’m closest too, though i’m close with all of them. i feel that being the oldest i’ve often taken on the role of protection towards my siblings. i’m the sibling that they come to if they have friendship problems, bullying or need advice, that has always been my role. on top of that though, my parents both work big jobs (they are both lawyers) meaning it’s me and the kids most days after school. my parents drop us off at school, my grandma drops us home and then they don’t get back until gus & leelee (my two youngest siblings) are asleep. most of what is between that falls on me, which i don’t mind but i definitely know some kids would.
[name_f][/name_f]i think you should always prioritise the children you already have, but do what feels right for your family 