This site let's you map out public travel. Ratp.fr. Note that using a bus and train requires two different tickets. With the metro you can transfer but assume there will be lots of stairs if that is an issue with your Mom. Make sure with the RER C that you board the right one as it branches and not all trains go to Versailles Chateau Rive Gauche -- the electric board over the track will show the stops and so board the one going where you want to go. This is not difficult; you just need to be aware so you get the right train.
Normally you could walk the half mile to Invalide and pick up the C there OR take the 13 from Clemenceau near your apartment to Invalide to pick up the C. But it depends on the day as there is weekend work on the tracks sometimes and that might make a more convoluted route necessary. The C is often down in central Paris for one reason or another.
You could also take a taxi to Invalide and get the C. RERs have escalators and elevators -- metro stations rarely do or have them only partially and often they are out of order if they do have them. Although RERs have elevators, the transfer from a metro station to the RER may not -- you might have to leave the station to find the RER entrance with elevators which might trigger the need for a new ticket. So if you take the 13, you may have stairs on transfer; but if you take a cab to Invalide you should be able to find an elevator to the RER C if it is running.
Always check with the Ratp.fr app first to make sure things are running when you wish to go.