Having our aunt call us cousins instead of niece and nephew would be kinda weird sooo don't do that. This kind of situation happens here all the time, especially in my family with all of the differing timelines. You'll get used to it.
We're kinda not so close in age? Mom said you were only 16 and I'll be 20 this year but I can't say when cause spoilers. I won't say no to being spoiled though! Spoil away!!!