Hello Bob, I can give you two veiws on Malta at this time of year, althought we have not been at Christmas we have been for New Year.
1st the weather the first time we went we had good weather for most of the week, so warm I was in a tee shirt most days but not that brave I went in the sea. The next time we had rain more days than we never although this did not stop us from going out and about to see the sites, so as with most places in the Med you take your chance with the weather.
As you already know Sliema is a working town as well as a resort so most things will be open (shops/bars and restaurants) they could well all be shut on Christmas day as in line with most other places, one word of warning if the buses are running Xmas day make sure it is all day, one time we went to Valletta on New Years day on the bus but they stoped running at noon and we had to walk back to Sliema, although this was no hardship I am glad we never went to M-dina that day.
So all in all I would say if you like Malta go for it, just remember that they do have a winter as well, and take a jumper and wet weather gear just in case.
Dave