Since you are asking your wheel to carry a bit more than an average load, you should consider a wheel with more than the average number of spokes.

If you go to a rear wheel, that is well built, on a 36 spoke hub, you should be OK. If you want to go to 40 spokes, you will be better off, but your options for hubs become much less. Both my daily commuter and my utility bike have 36 spoke, hand built wheels. Between them I have close to 20K miles, with no wheel problems. Wheel has a Shimano 105 hub and a Mavic Open Pro rim with DT double butted spokes., the other is a Phil Wood hub with a Mavic 719 rim, also double butted spokes.

I have a 40 spoke rear wheel with a Phil Wood Hub on the back of my tandem, it would probably support my mother-in-law with you in her lap .