My (F42) sister (F39) told my son that she is going to be his mother because I have terminal cancer.

I have Stage 4 breast cancer. It's terminal - I'd most likely have 2 years or less left. Because of this, I'm spending as much time as possible with my son who is 14 and my family. *Just for extra information, I am a widow - my late husband was killed by a hit and run driver years ago.*

A week ago, I overheard a conversation between my sister and my son. She was telling him that since his mother (me) was going to die, she will be his mother. I was horrified. My son strangely seemed to agree, but he was in tears (my sister wasn't).

I am quite close to my sister so I know that she really loves my son. She has never had a child herself and she kind of treats my son like her own. I had a problem with it, but she told me she was just loving him, so I put it aside. I didn't know it would escalate to this.

I confronted her a few days ago after careful thinking. I asked her why she told my son that without asking me first, and she said "but isn't that the truth?". To be honest, I am not sure if I can trust my sister with my dear son - I never really thought she was fit to be a mother, especially not to him. I said that it would be my choice, not hers. She agreed, but she said she was the most suitable person to parent him. Her exact words were "I will take care of him well, you wouldn't need to worry. I will be a good mother to [my son's name]."

I don't want my sister to be my son's mother. I am not on my deathbed but I am nearing my end. It just really breaks my heart to hear my sister talking about how she'd take over my "position as a mother" when I go.

My sister said she'd give me space to think. What do I do? I know I shouldn't be on Reddit for this but I have exhausted the people in which I can ask advice from. Thank you.

tl;dr - I am dying. My sister said she would be my son's "new mom".