Can I reject mother sponsored Green card and accept sister's?

Hello, both my mother who is a Green card holder and my sister who is a US citizen sponsored me for a Green card 10 years ago. I was over 18 at the time. Now my mother's sponsorship is being processed and I need to make a decision before 1 year is up. But I am in no position financially or professionally to go to the USA so soon. So I want to reject this and wait until my sister's Green card sponsorship comes through in about 3 to 4 years time. If I reject the first (from my mother) will there be a risk in getting my sister's Green card sponsorship later?
Thank you
 
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