I feel like I'm not playing with my twins enough

I have 3.5 month old twins and I usually play with them by talking to them or playing with their hands and feet. I didn't think they'd be able to understand more, but my in laws came over yesterday and brought a toy for them, and they just lit up and started smiling and kicking, now I'm feeling guilty like I should have been doing that more. They also read to them and they seemed to enjoy that as well when I have barely read to them. I know it's beneficial to read to them but its so hard for me to grasp that they can understand or like it at this age. Anyway I'm worried that I've been holding them back by not stimulating them enough. Thoughts?