I think perhaps OP would feel a bit differently if she or her DH had also been invited. Why not get yourself/DH a ticket and go too OP? Some of us just don't like the thought of our children being without a parent if a difficult situation arises - there's absolutely nothing wrong with that!
It's not just the thought of a horrible death in an inferno. The landings can be tricky and not everyone can hold on tightly enough to stabilise their own weight - causing injury to themselves and others. To be honest, I'd be more worried about the MIL than your son.
This is an extract from a report on an accident in Derbyshire in 2011:
As the flight progressed towards the high ground, the windspeed increased and the pilot elected to land. However, he was unable to do so until the balloon had cleared the obstacles on the ground. The passengers had all been briefed and instructed on how to adopt the landing position and this was called for by thepilot during the approach. The injured passengers had all adopted the landing position but the impact with the wall was immediately below their corner of the basket and this caused the injuries. The female passenger, who broke both her legs, was 64 years of age and had no medical history that may have contributed to her injuries.
In a report on a hot air balloon accident involving G-KTKT, published in AAIB Bulletin 8/2010, a reference is made to the number of ballooning accidents which have resulted in ?serious injury? and that in a significant proportion of these reports, the injured passenger was described as ?elderly?.
www.aaib.gov.uk/cms_resources.cfm?file=/Cameron%20Z-275%20hot%20air%20balloon%20G-CBZZ%2002-12.pdf
Extracts from the safety regulations:
"Balloon landings can take place at unprepared
sites and may occasionally be bumpy for the
occupants, especially in higher wind conditions if
the basket tips over and drags along the ground.
At present, not all commercial balloon operators
make passengers aware of this, either at the
booking stage or prior to a flight. Therefore, it is
recommended that the Civil Aviation Authority
require all commercial balloon operators to make
prospective passengers aware of the varied nature
of balloon landings so that they can make an
informed decision as to whether or not to undertake
a flight. (Safety Recommendation 2010-052)"
"Operators should ensure that prospective
passengers are aware that ballooning is an
outdoor activity, normally occurring in the open
countryside, requiring a degree of fitness and
activeness from participants."
Virgin do not allow anyone under 16 unless they are accompanied by a responsible adult, and frankly a 70 year old would probably not fit the bill.