Oh, if I could atone for my participation in this heresy. Seriously, it boils down to the coincidence that both Jill and the MC had an older, wealthy, female relative die tragically early in life. But the ages don't match. Conservatively, if Jill is either the same age to 2-3 years older than the MC, then how can she in the flashback be at least 10-12 years old and have memories of her sister?
The other idea is that the house in the photo with Lynette's friends is Jill's family home. There are other theories that are more reasonable than the Aunt Jill theory.
Mostly, it's a joke at the expense of those of us who like Jill. But that's my two cents (8 cents with inflation).