Rambles ab Rosa & Nadia down below...
When she was younger, maybe in her teens, I imagine Rosa used to write a lot of short stories when she was younger, likely centered around adventure and whatnot—I think she probably got inspired by Verne and Stevenson; maybe even the Lewis and Clark expedition? She used to read a ton (and she still does!) Either way, I think this was her gateway into journalism once she realized theatre wasn't for her.
As for Nadia, she got into writing due to all the crazy stuff that occurred throughout her life & the things going on around her. It probably started off as a coping mechanism for her before it turned to her actually writing and creating her own characters, plots, etc... I imagine it was mainly the events of MM & PL3 that influenced her writing, maybe even the housefire she survived. I feel like that especially manages to creep into her writing and she doesn't even realize it until Bibi points it out.