We don't know much about Jade.
The MC met her when she was a cashier at a convenience store/gas station but no reference to what day of the week (school day/weekend for example).
I feel like she is a little older (1-2 years) but that could also be because the triplets were sheltered by Gracie and homeschooled for years - which makes the triplets more innocent 18-year-olds compared to their peers.
I think we are still far too early in the story to see where/if Jade fits into the MC's life with the triplets.
There is no way to guess how many updates it will take to flesh out this story enough to see.
Cheers!!