I like Mary Roach books, though they're not sci-fi; they're nonfic. I recommend all of them unreservedly EXCEPT Spook because of the subject matter and Mary's approach to it. However, it's a far more interesting read when you need to refresh your memories of the "spiritualist" movement like I did while reading Things Half in Shadow by Alan Finn. (Recommended, by the way.) If I had to pick one, either Packing for Mars or Stiff would be my first choices for a step-mom.
Right now, I'm reading a lot of nonfic and fantasy (Dresden Files series, though it's more like ... fantasy-noir) with a few author binges (Neil Gaiman) thrown in for fun. Not sure if Gaiman is your family's speed, but my current Gaiman faves are American Gods, Coraline, and The Graveyard Book. The last two are marketed as YA, so YMMV.
The only sci-fi I've read is Starship Troopers by Robert A Heinlein ... and some Michael Crichton (Jurassic Park and The Andromeda Strain), though I've been meaning to check out Ursula K LeGuin (although she doesn't consider her stuff sci-fi) and Octavia Butler for quite some time now.
I recently read Kindred by Octavia Butler, but that's less straight-up sci-fi and more time-travel/historical fiction (but not in a feel-good way) and I do recommend it, but not as a Christmas present!
For your SIL, what about something you enjoyed reading for fun? When I want fun, I turn to fluffy books, like the Shopaholic series by Sophie Kinsella or the Heather Wells mysteries by Meg Cabot. Those might be decent bets without knowing your SIL's taste - they're short and pretty funny.
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Liz
DD (3/2010)
"Make mistakes! Get messy!" - Miss Frizzle