Why to "jump in my grave" and what does it mean in this sentence?

Question

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.

Answer 1

It simply means taking someone's former possessions or positions too hastily. The phrase is used colloquially and does not carry the negative connotation of "graves".

In your context, the older sister acted hastily by requesting to move into the room when the younger sister hasn't even moved out just yet.

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The phrase originated from a poem by Paula Meehan -

Would you jump into my grave as quick?
my granny would ask when one of us took
her chair by the fire. You, woman,
done up to the nines, red lips a come on,
your breath reeking of drink
and your black eye on my man tonight
in a Dublin bar, think
first of the steep drop, the six dark feet.