I appreciate all your feedback and valid points. And, I have accepted the invite after reading your feedbacks. But, I have met, held, cuddled, fed our little GS, so going there is only a formality that serves no purpose as we, her parents and us, have already celebrated our GS's homecoming 3 weeks ago.
My DIL and I get along very well. She has lived with us and joined us on many vacations. She is a wonderful mom and never seizes to amaze me!
And she isn't really the problem. My husband and I know her parents well, too, and, let's just say, in order for us to maintain an amicable relationship it's best if we keep our distance from them. I know she does not do them intentional, but, unfortunately can be quite domineering and condescending at the same time. It's virtually impossible to have a conversation with my DIL without her mom speaking on her behalf. Which exactly why I prefer not having her around when I host something for my son and his wife.
Here is another concern. I am old school, as are my friends and family members. We still have the believe that wedding/baby showers are a gift that's bestowed on them and a privilege and not a must/expectation. And we really struggle with the new trend of new parents/parents-to-be planning and organizing their own showers, telling people what to bring, and not bothering with "thank you" cards (for which I still embarrassed about that my son and her never sent out any thank you cards for the bridal shower--which I hosted--and wedding gifts). As soon as my DIL was pregnant, she started telling everyone how many showers she going to have/plan, who is going to give her a baby shower, etc. I was part of that picture and felt like I didn't have a choice. I was never asked. It just seemed to be expected of me. So instead, my family pooled together and bought them some of the more expensive necessities on their registries and gave those to the couple at Christmas when we were all together in lieu of an actual shower, which is what we told them.
Since then, many of our close family friends, whom my son grew up with, have passed on gifts through me to the couple, while they were in the hospital, to help them out. It is for that reason why I'm planning on hosting and paying for a "sip 'n see" (aka meet 'n greet) because they haven't had a chance meet our little GS yet and are dying to. That is why I don't really see a reason to have to invite her mom.