The mothers as friends and Jenn going to school with her sister sister both work perfectly! Though Sadie would always be the older sister Lusitania was sighing about- it's very much a case of the pampered older sibling that wins just by being born and doing everything first, while Lucy worked hard to earn credit and attention.

She could always be a second cousin twice removed (or closer, whatever) with Blackwood, as well? I always picture the rich Capitol families like European royalty. Everyone's related somehow.