Well, If the drivers airbag had deployed without your mom wearing her seatbelt, she would have injured or killed.
In a MB, if the belts are not buckled, there will be no SRS deployment from the front. If you hit the steering wheel because
you were not buckled, you do not want to meet a hot airbag coming out at 200mph. Yowch!
Most modern MB's like the 01+ C class have realy neat front
crash boxes. These absorb a whole bunch of energy and make repairs easier than cutting on the frame. This usually will cause
even a severe wreck to not need SRS deployment.
That is most likely why your brother's air bag didn't pop either.
The car did such a good job protecting your family, SRS wasn't even needed.
One of my clients was driving his 2004 E320 when some drunk
crossed the centerline and hit him head on. The impact was a combined 80-90 mph off set. The other driver was killed and the
client had only his seatbelt tension up. He walked away un-injured. We checked the data from the black box to see if there
was a fault. Nope, the car never reached the air bag deployment threshold. The cars are that good at protecting occupants.