From End of Greys
Rachel refers to her grandmother's sister's husband as her uncle. That would be her great-uncle by marriage. It makes ALOT more sense that Liam and Roger are also her great-uncles via being her grandparent's siblings, NOT Jean
ETA:
The narration is a bit confusing but I believe it means Julia bonded with Jean's big sister. Brian and Julia took custody of the kids after Sara died which makes more sense if they were a married couple, not uncle and niece
What I can deduce is Liam, Roger and Brian are Jean's uncles and Julia is Brian's wife, making her an aunt by marriage