As you can see from here:
http://www.mtrcorp.com/eng/train/ae_time.htm
the last train leaves the airport at 00:48 so no problem there.

You can use the mapping service here:
http://www.ypmap.com/en/viewer.asp?m...ce=LocationMap
to locate the MTR station nearest to your grandmother's home. Kwai Chung doesn't have an "e" in it as normally spelt here.

If it is actually in Kwai Chung Estate then the nearest MTR is Tai Wo Hau, and you can use the MTR journey planner here to work out how to get from the airport:
http://www.mtrcorp.com/eng/train/intro.html
For the airport to Tai Wo Hau you have to change twice at Tsing Yi and Lai King onto the Tseun Wan Line MTR line. The last time you can leave the airport would be about 00:15.

For the region Kwai Chung it will be one of the MTR stations between Lai King and Tai Wo Hau on the route map here:
http://www.mtrcorp.com/jplanner/imag...s/mtr_map2.jpg

Hope that helps.