Before I get to the main question, here's a bit of background. My fiance and I have been together for 5 years at this point, and will be getting married in a year and a half. We're both from blended families, though the circumstances are different, and it's causing some anxiety on my end. His parents divorced when he was in middle school. While they are on speaking terms with one another, they don't regularly interact. He's very close to his dad, which has strained things with his mother a bit. By contrast, my mother and my stepdad - who I've always called my dad - have been together since I was still in preschool. Aside from that, I had a very traditional, filial-piety-esque upbringing, so I'm incredibly close to both of my parents.
Following his proposal, my parents were thrilled, his father was thrilled, and his mother was meh.
I don't want to shut his mother out of the wedding planning, but I'm not sure whether I even want to involve her. My parents are helping with the ceremony and reception, his dad is helping with the rehearsal dinner, and he and I are paying for or making the wedding attire, invitations, and gifts. She wouldn't have a real duty, and I don't want to ask her to do something when it seems she couldn't care less. However, I feel somewhat guilty about not trying to reach out to her. He doesn't seem to bothered either way ("I just want us to be happy, babe."), but I don't want to be dealing with this fallout years down the road. I don't dislike the woman, and I don't want to add strain to her relationship with him.
I realize it's a long shot, and a long question, but does anyone have any advice here? Should I just tell her "This is what's happening" or should I try getting her input? Is it a major faux pas not to include her? Any thoughts would be appreciated.