How to deal with a mooching family member?

My grandpa is 89 years old and is widowed since 2008. His only son (my dad) passed away in 2005. My sister and I are the only immediate family to care for him. However, my sister does not even contact my grandpa or myself to check on his well being. My 23 year old nephew lives with my grandpa and always talks grandpa into giving him money or pay his bills. My grandparents raised him so he knows how to guilt trip grandpa into anything. I am POA but my grandpa has not been deemed incompetent. He is developing memory loss and does not understand how to control money or basic mon thy bills. My grandmother handled all the bills. I have tried over and over again to get grandpa to understand the nephew is draining his bank account. I am lost on what to do. The stress is killing me. I become the enemy when I try to voice my opinion. I don't want to see him lose his 80 acre property because of a thankless brat. This has been going on 7 years since grandmother passed. Any suggestions is greatly appreciated.