Sorry to hear about your situation. I've had to do the same several times since about 12 and never gets easier. They are family members. Our 11 year old Lab has about another year, maybe two. It won't be easy for me but it is going to really hurt my kids, 8, 10 & 12 when it gets here. When it 's time it's time. You don't prolong it. It's worse if you do.
Good luck.
Good luck.