Well, the Thunderbirds Wiki calls her Maureen, but other sources disagree with the name. All do, however, agree she died.
According to Wikipedia, we learn there are multiple causes explained for her death:
His wife died prematurely, commonly accepted to have been due to complications with the birth on 12 March 2044 of her youngest son, Alan. A non-canonical 1993 comic strip has a different explanation [She and Jeff's father died in an avalanche], while a novel published in 2008 had her pass away as a result of a road accident.
- She died after complications in Alan's birth
- She passed away in a road accident
- Both his wife and father died in an avalanche
But here we are told his wife's name was actually Lucille (Lucy), although providing the reasons of an avalanche or complications at Alan's birth.
It would seem most likely that either of the above are possible, although the most likely would be the avalanche, hence the inspiration for creating International Rescue, which is another question altogether!