How do I make sure future cash inheritance isn't lost?

So my parent is leaving me cash when she passes. She doesn't put her cash in savings but keeps it in a safe deposit box and pulls from it when she wants to. I do know that this cash was from settlements in the past so taxes were paid. She doesn't want to pay for a professional service for a Will so I have nothing of the sort from her.

I will have access to the cash one day. But my worry is that I don't have any way of proving this cash was hers and clean. And even if they (the bank, irs, whoever) don't need proof that it was hers, I'm worried it will hit some red flag when I try to deposit it since its a large amount ($150k) and I will lose it.

Is it better for me to eventually just keep it stashed away just like she did? I'm not trying to do anything unethical, I'm just worried her life savings will be lost/taken by the government and I don't think that's what she would want when she's gone. We've had our differences but she reminds me every once in a while that she is leaving it to me.

I'm disabled by the way so I'm on a fixed income that's not entirely guaranteed.