I don't think she specifically said 13. If you can find that, let me know. It's more or less been an assumption based on the dates we have. We know it happened before David met Ophelia, and we know Ophelia had Brenna at 15. If David & Ophelia were a similar age, then that means David was
at most 15 when he was having sex with his mother.
Based on the estimates we have, it seems Nora is the only one who regularly had sex with underage kids (Drew from age 13 or 14, and David from age 13-15). If the threesome image from 2004 is similar to what they'd normally do, it's likely Keira was involved in some of those encounters as well. She would've been underage until 2002 or so.
Hunter didn't seem to have sex with Keira till she was at least 17-18. Lucia would've already been 2-3 years old by that point.
With regards to your second question, yes the time frame is meant to show her battle with cancer from 1998-2002, which resulted in her death.