From the POV of the present buyer...
Admittedly, I didn't spend much on them because I don't have much in terms of money this Christmas, but I got gifts for my metamour and her and my bf's kids this holidays. I wasn't trying to buy attention or affection and they didn't get me a thing, and that doesn't bother me at all. I got them presents because despite having limited in-person contact with them, they are a part of my life and I always get little things for the people in my life, if I see things I think they will like. I didn't get a thank you though, which bothers me, but that's not important to this thread.
Please consider that she may have bought gifts simply because, with no expectation of return. A thank you may be all she would want. If they make you feel super awkward, ask your hubby to tell her you're less into gift giving and as a family they are unable to spend similarly in return, and I bet she doesn't care one bit. It sounds like she is trying to make you comfortable with her being in her husband's life by showing you some kindness - please give her the benefit of the doubt unless you are convinced there are problematic motives.