Of course anything I say here is all speculation, and mostly depends on your meta-game and the type of player your opponent is (e.g. WAAC's).
But assuming he's a half-decent player, the things to look out for would be:
1) some sort of Min-max setup of nightscythes each with X5 Necron warriors and a harbinger of the storm. He might have one or two of these and would act as semi-suicide units and go around poping your vehicles.
2)wraiths, lead by a destroyer lord. This will probably be the most annoying unit in his army for you or your sister. But as you say, you will probably sit back for two-three turns and with your sisters, 'Eat Hot Plasma Death-configuration', he'd probably go after the dark angles, because all the plasma's low AP weapons won't mean a thing against our inv saves.
3) Doomscythes, these have the same armament as the nigthscythe with the addition of the, 'death ray' (yes i promise its called that). this thing will most likely ruin your day, As it's a very high strength very low AP weapon. so don't keep all you vehicles and infantry units too close together.
3A) In fact if you see any sort of flyer's in his army, and he may have a lot (we can take 9+). Make sure you ask which variant it is, because they both look very similar to each other and could be quite confusing at first.
4)Ghost arks, if he has one of these full of warriors (up to 10). he'll use these as gunboats pumping out 30 Gauss shots in rapid fire range, then late game they'll go after the objective. Once he gets them there, you'll have a very hard time getting them off, so these are your priority targets.
5) Mindshakle scarabs and warscythes. I won't go into detail (got to leave you some suprises), but sufficed to say, - stay
AWAY from these things. on a more practical note, take an extra pair of trousers with your because if your IC does face off against these, you might find yourself with funny brown stains on yours all of a sudden.
6) Annihilation Barges. He'll probably have at least one or two of these because there just made of pure awesomeness. they do alot of damage for there low points cost. he'll' most likely use these as (primarily) anti-flyer, and (secondary) anti infantry.
7) just about everything else in the codex.
If you both team up against the Necrons, you'll probably have an easy time of things. But if I were you I'd go after the Dark angels for the
'Moral Victory'.