Marley and Me is an excellent (but sad) story about a dog if the 13 year old is at all into dogs.
Do they have any upcoming Disney trips? If so, besides the obvious Birnbaum's for kids for the younger one, there are a lot of Disney trivia books out that would be fun for the older one to read to the family.
Any of the books by Andrew Clements (Frindle, Room One) are excellent and could probably be read and enjoyed by both girls if they haven't read them yet. They're mostly about kids in the 4-6th grade so it would be "fun" reading for the older one, and a bit more challenging for the younger one.
The Peter and the Starcatchers/Peter and the Shadow Thieves series written by Ridley Pearson and Dave Barry are really good. They tell the prequel to Peter Pan and are well written, but very long (think Harry Potter). Definitely for a 13 year old (again especially if Disney is in the future); probably a bit high for an 8 year old.
Magic Tree House - the entire series - for the younger one. All of my kids loved these series and we still get them as they come out, even though they're quite a bit too old for them now.
I'll try to think of more!