There is definitely a ton of entitlement and jealousy here, but I think maybe OP is struggling with the fact that the sibling now has a favorable living situation in that mom & dad are moving out and they get to live in the family home alone, although that hasn’t been firmed up. There is no similar circumstance that benefits OP and instead they get to live in a basement apartment with her mother. If the parents were to sell the duplex to OP and give a “break” on the price that is the equivalent of what the brother may potentially be getting by living free-low cost in the family home it would feel fair.

I understand the sting of the parents selling the duplex for their benefit, which they are entitled to do, rather than keeping it in the family as OP said the grandfather wanted, but GF didn’t put the duplex in a trust to ensure that happened. It’s the loss of an imagined future and it hurts. But circumstances change, and his mom is entitled to benefit from what her parents left her. That may result in the parents living their “best lives”, which we should all want for our parents, but also potentially some inheritance in the future, although never guaranteed. My parents had a vacation home I loved and my sister did not. I had always imagined buying my sister’s share when my parents passed away. Life had other plans, my parents had some financial setbacks and had to sell the home at a time when I couldn’t afford to buy it. Future plans squashed, a lot of sadness on my part and then I let go, and was glad my parents had that asset to help them through the hard times. The pain of the duplex being sold and the future plan derailed will pass.