so sorry for your loss... i don't think you and your family are being unreasonable.
three weeks before ds was born my dfil died v suddenly and my sil insisted on bringing her 18mnth old son to the funeral. nightmare. we were devestated and had to run about after this kid which was a total nightmare. i know, i know, an 8 month old is less likely to run about, however, i'm making a point, i guess. we should have been focused on my poor mil etc, and had to change the child's nappy, get him food etc, at really awkward moments. And generally because sil said she was too upset to do it herself. Can totally understand her being upset, her d dad had just died. however, it wasn't fair to make the rest of us look after her child when she did have alternatives available.
three weeks after ds was born my very d papa died v suddenly. we had to leave my ds with my mil for four days (although we came back at night) as there was no way that i, or my dm, or my d grandmother or rest of the family could have coped with a three week old baby in the middle of everything. i made the right choice. my ds didn't suffer, i didn't have to worry about trying to comfort people and panic about getting a baby fed. but, i guess, i made that decision myself.
personally, in my experience, i don't think you're being unreasonable. others will prob have a different view, but i'm allowed to have my own experinces and opinions
you have to do what is right for your mum and the rest of your family. as mumsnetters we can offer opinions, but we're not in your situation, so it's really up to you to go with your instincts.