If my memory serves, AVCHD is MPEG-2 transport stream, with video in H.264. A peek on wikipedia reveals the audio is in AC3.
Do you have any DVD authoring program? (see a list here
http://en.wikipedia.org/wiki/List_of...g_applications )
The next thing to do is to check whether the app can open the AVCHD files. A quick test can be done by trying to play the file in Windows Media Player. That'll tell you if you have the MPEG-2 filters (for parsing the container/file format), an H.264 decoder (for video), and an AC-3 decoder (for audio).
If any of those fail, first thing to try is to install the software that came with the camcorder. It should have decoders for all 3.
If that doesn't work, try ffdshow-tryouts (
http://ffdshow-tryout.sourceforge.net/). Make sure AC3, MPEG-2 and H.264 are enabled both video, audio, and VfW interface (you'll get an entry in your start menu to configure each).
P.S: If the relatives you're sending this to have a PC, it may be simpler to get them to play this using VLC (
http://www.videolan.org). No re-encoding needed. VLC has all its codecs and filters built-in independently of Windows, so installing it will not make the file readable in any Windows app (including DVD authoring programs).