I haven't propagated lilacs from cuttings, but here's what Michael Dirr says in "The Reference Manual of Woody Plant Propagation" (an awesome reference book):

"Spring softwood cuttings are preferred. Cuttings are collected just as the flowers are starting to open and until the end of the blooming season. Treat with IBA (1500 to 5000 ppm depending upon cultivar) and place in coarse sand or perlite under a white poly tunnel or mist."

He gives a couple other successful methods that are used, but it's too much to type here.

As far as your mother-in-law's experience, I'm guessing either she stuck dormant branches in the soil in the fall, or she had perfect weather conditions (warm and wet) for rooting continuously for a week or two after sticking them in the ground.

Hi! When I moved to this new home 4 years ago I left a huge lilac bush that I put in 40 years ago. The orginal start came from my great grandma that she brought over on a ship many years ago. I just took some small starts from the bottom of the lilac bush that were in the ground. They are doing very well here in their new location.