Meaning in Context – Why Use ‘Jump in My Grave’ and What Does It Mean?

I could understand the person who said this sentence is that she is angry and accusing her sister taking (stealing) her own room. But why would someone compare her own room as a "grave"? Isn't it bad or unlucky to say something in this way? Or am I missing some culture gap? Or is "jump in to one's grave" just a simple collocation which I failed to grasp its meaning?

Here is the sentence:

I suppose with hindsight it would have been a good idea to mention it
to my sister first.

Three hours later she came bursting into the living room with a face
like thunder.

‘Would you jump in my grave so quickly?’
_{Me Before You by Jojo Moyes}

The context is :

The thundering sister is moving out with her son to university, and "I" being the older sister, has being living in a box room for five years, now that "my sister" is moving out, so "I" would like to move in the room, which is larger and originally mine several years ago. But "my sister" refused that because she may come back home at weekends.