Taper the foam, an electric knife works well for that.....or wrap first in muslin....pulled tightly to compress the foam at the front. Then cover with your upholstery fabric.

Do you have a picture of the chairs, or something similar? I just finished reupholstering a set of shieldback dining chairs for my dining room; I bought them from an antique dealer at a local flea market because I fell in love with their lines, but like your MIL's chairs, they were hard and uncomfortable. I removed all of the stuffing and prior upholstery (taking it down to the bare wood seat), added 2" high density foam, and covered that with batting and fabric. I probably should have used 1.5 inch foam but the fabric stores in my area only had 1 and 2" in the foam that I wanted. The thickness of the foam, paired with the shape of the chairs and the stiffness of my upholstery fabric caused me to have trouble pulling the corners taut; so I had to fold & crease the corners (I've never had that problem recovering prior dining chairs), which annoys me because it makes them look less professional.

Like Lindac suggets, the electric knife worked well to cut and shape the foam. The front of my chairs curves, like a bowfront chest, so I traced that shape onto my foam (using the bare wood of the seat base), then used the electric knife to cut in the curve. The electric knife was key; the foam was too thick to cut the shape with scissors, and my utility knife gave me uneven edges. Once I switched to the electric knife it was smooth sailing.