rydercup, it's a long story, and I don't want to worry your SIL but in a nutshell, she carried on projectile vomming from birth to 2.5/3ish. She had various tests (barium swallow, x rays etc) and had various meds and different feeds, but the hospital could never find anything wrong structurally, other than her tummy took slightly longer than normal to empty, and her pharynx is slightly narrower than normal. Eventually, in despair, I took her to a cranial osteopath, and in 3 sessions he sorted her out.
The thing that upsets me about it is that I was telling the GP, HV etc for ages that I was worried, they kept saying she'd grow out of it and that most babies can be 'sicky' at times (I just felt 2.5/3 was not a 'baby'). Because she would be sick 4/5/6 times a day, her mouth was fairly regularly full of acid, so as her teeth came through they rotted. She has just started reception and only last week finished a very traumatic year long course of dental treatment to patch up her teeth (couldn't save the front 2 they had to come out).
If you SIL's little boy keeps being sick, or starts doing it when he's not in the car, I would recommend cranial osteopathy!
Sorry about the mammoth post!!