I can see this as touchy area....I might go to the MIL family and feel out how they would feel about you upgrading for her. I don't have an older second machine, but lots of folks love these. She may find a bigger machine more than what she wants.
I am reminded of when I was a kid and wanted to get into stamp collecting, for Christmas my mom got me a stamp album bigger than a dictionary and a bunch of loose stamps.... it no longer felt like a hobby, it felt like homework. But I had to do it, after all my mom got it.
So now I often look at big purchases by the amount of guilt they will leave around if I don't use it. Maybe she will find more machine intimidating. Is she happy with the basics she has on this machine....if so, get the feet and spring the extra money you might spend on the machine and buy some fabric or a gift card for fabric and notions.