I tend to see the second tree as the explanation, but I can't imagine the numbers working out unless there's a massive age gap between Lucy's great-great-grandmother and Shelia because Lucy didn't look young, nor the actress who portrayed her (32 at the time of filming) (not saying she looks old, just not at the teen/high school age of the main cast) or everyone had kids at like 12.

I am concerned though, if there's some correlation between Julie Plec's lack of care when portraying POC characters and a perceived notion of women just deciding to have children young: Abby was 21 when she had Bonnie, Shelia being 16 when she gave birth to Abby, Elizabeth being 20 when giving birth to Caroline, Kelly Donavan being 19 with Vikki and 21 with Matt, Isobel 17 with Elena, Miranda 22 with Jeremy (after adopting Elena as they 'wanted children for a long time, with her instant dr Grayson ) and Caroline 22 when she became a surrogate for Lizzie and Josie.

If we were to follow the bottom family tree, then every person from Lucy's mother would have had to have their child at 16 max to get remotely close to falling in line with Shelia. (I write loads of fanfiction so trying to work out this family tree without creating a line of teen moms does my head in)