Bill, I have a good solution for you: Suggest your family to walk the last 100 km not from Sarria but from Lugo
That's another starting point which becomes more and more popular every year. Pilgrims start from Lugo (which entitles them to receive Compostela in Santiago), walk 50 km on Camino Primitivo to Melide and from there they follow 50 km of
Camino Frances. Lugo is a beautiful Roman town surrendered by fantastic stone walls and you can walk on top of them, I am sure your family would love it!
Lugo walls
Lugo walls walk
This arrangement would let you follow Camino del Norte all the way to Villaviciosa, take the turn to Oviedo and then follow the Camino Primitivo until meeting your family in Lugo and walking together to Santiago from there.
Well, if it doesn't work, I agree with Polar Bear's advice (above) on walking from Lugo to Sarria - the distance is 30 km.
There is a local road LU-546 that connect both towns. What's more, it seems like it is fairly traffic-free, as the parallel highway took over most of the traffic from it. There is a little town more or less half way between (Pobra de San Xiao) to take a rest.
This is what it looks on the Street view (I am not sure if the links will work):
LU-546 Street View
A bar in Pobra de San Xiao
Buen Camino!