That's true! Siblings petition take forever... If you are USC and petition your mom, she will get the GC in less than a year.
It doesn't matter that your uncle already applied for her. When you apply now, it will cancel your uncle's.
<From my own experience> My sister petitioned me and all lawyers said i will take forever (at least 8-10 yrs, just like you said yourself). She petitioned my parents 2 years after me and they got their GC (whole process less than a year).
Good luck!

