I don't. My parents. Their property.
This. I just cannot think of my parent's money as my money, even theoretically.
I don't think of it as anything but my parents' money, for sure. But they've also been very clear, for many years, that they are planning to leave money (with specific $ ranges mentioned) to my sibling and myself. They also have asked, multiple times, if there are specific items either of us want. For example, I'd like to have my mom's charm bracelet and the hope chest she got from her parents for her 16th birthday, both of which are anchors of very special childhood memories to me. It actually thrilled my mom when I told her that, to the point that I suggested to my sister--who is much less sentimental and demonstrative--that if there was an item she even kind of wanted, she should mention it because it would bring mom mom a lot of joy. But that doesn't mean I think of that hope chest as mine. If my parents decided to sell it tomorrow, they'd be well within their rights to do so, of course. it is their property.
Maybe my family is unusual in that we have fairly candid conversations about this stuff, and that helps. Having a sister who worked in a death-related profession probably helped with that. But we know who the executor will be, what is in the will, the approximate value o the estate, the various organizations where my parents have accounts, their basic wishes for the money, etc. None of that makes me think of it as mine in any way, or feel I have any say in how it is spent or what my parents do with it (dad has a decent chunk in an actively managed account! The horror!). It just means they wanted to prepare us so that when the time comes, we have fewer details to figure out. They put this in the same category as telling us their passwords and code to open the garage. That doesn't mean I feel like I suddenly own their home or am entitled to log in to their email. It just means I have info that might be useful someday.
I'l add that dad is very proud to be leaving money to my sibling and me. I think it would hurt him a bit if I said I didn't want his money. Acknowledging that it is a huge gift to me has value to him, just as telling my mom that I fondly recall sitting with her, over and over, and asking her to tell me the story of each of her charms on her bracelet had value to her.
Even if I planned to donate every penny of an inheritance immediately, or take the bracelet to a pawn shop for melt value, telling them those things would be hurtful. Giving is a an act that has great value to them, so I honor that by expressing my appreciation.
I know it's not that way for every family, and I am fortunate that there are no complicated family dynamics and everyone is respectful and not motivated by greed. But for my family, acknowledging that there will almost certainly be an inheritance, and that it is valued and meaningful, has value and meaning for my parents, too.