I know that it is hard to act according to advice given by others if you have someone else who is ''technically'' the main owner. (I'm not the solo owner of my boys either) But if he's the only one who is telling you not to bother the mum, and at least FIVE OTHER PEOPLE have advised you to put the kit back with its momma and that they would be fine and best off with their littermates, then why is this ''gramps'' person stopping you from acting according to the advice given by other, more experienced rabbit folks? Say a couple words back to him, maybe show him this thread of people that have given you great advice, and if he doesn't speak english, translate it for him. It's exactly what i've done and it has helped me many times. I can't say for sure, but i am more than certain that the well-known folks over here are way more knowledgeable than your grandparent is.
You have been told multipe times now that the kit would fare best with their momma and if she isn't dead or anything, you shouldn't really be the one taking care of the kit. As mentioned before by other members, if you place the kit in the nesst when mum isn't looking, she will most likely just keep on doing what she's got going-caring for the kits. She wouldn't really single this one kit out, and she isn't going to eat the little kitling. She has other, more important things to do than consuming her young.
No matter what argument you bring up, the kit would still do best with their mum as no other thing or person is gonna be better than care and nutrition from their own mama.