I'm sure it will be ok. I'm sure your dp's grandmother would love a little hold. Just make sure she is sitting down and be close. I have previously read you have issues with ur MIL but without sounding disrespectful "I" think you are being a little unreasonable with your dp's grandma of 90. She probably loves your son greatly! Take a photo too cause ur son may appreciate that when he is older. Not everyone has great grandparents. I know that things like this can seem annoying specially with inlaws you don't get along with.