That's what I got from it too, this was the last time Jill ever saw her sister alive, and she looks what, eleven, twelve ish?Yeah it is very much a farewell.
Also, on your theory about 'Jonathan', I always had the impression he was from or the leader of the support group / therapy group that Jill went to after Lana's death, and that's where she met Bella, who was grieving her <redacted> husband James (and/or possibly dead child)?
They bonded over their loss, and Bella became a sort of surrogate big sister to Jill?
But what do I know.