I agree with your sister - but we are all different and our acceptance of risk is different.
Generally the rule is you shouldn't do anything that you weren't doing before you fell pregnant with regards to exercise. So I completed a pretty tough and rigourous preseason for netball this year, so I had no problems playing netball competively this pregnancy till I was 12w. I umpired to 19w.
So my body was used to it - it didn't have any problems keeping on going, and when I did have problems, I sure knew about it!! (Strained ligaments, out of breath etc).
I personally wouldn't do anything that exerted extra forces on my body like skiing, water or snow, fast boating or an extreme sport. Thats just a little risky in my mind - but everyone is different!

ETA - when I had my m/c my GP told me a friend of hers who is a dr lost her baby at about 10w the day after a tough netball game. Its pretty true that you can't cause a miscarriage by playing netball (unless you get a good thump in the guts in your secnd tri) but apparently even that dr couldn't stop blaming herself for it. You just gotta be sensible andknow when enough is enough. Everyone is different.