... and, even with your parents data you can't tell in some cases such as when all three of you are heterozygous (i.e. one of each). For example, if you are AG and mom and dad are both AG, you don't know if your A came from mom or dad since they both have one, and the same with the G.
The other option for phasing data is by statistical analysis using frequencies in the general population and other fancy math which is what ancestry.com apparently does for matching purposes (but not in the raw data we get). I'm not sure if there are any third party tools that can do this - it sounds like a big deal (and proprietary of course).