Jo-bel

You and your mom should each file I-130s for your siblings.  
 
This way if one petitioner passes away, while that petition dies with that petitioner the other petition  lives. 
 
Your mom cannot petition her grand children. Your sisters’ children are derivatives.

Your mother petitioning them is likely to be the fastest option unless something very weird happens.
 
Downside is that they should not marry. If they do the the timelines change a lot and best for you to petition them now.
 
Now you can of course petition them as well, you could have done back when you naturalised.
 
Children are derivatives so they are not petitioned, whether they will be included depends on age when this becomes possible and how long the I 130 takes to process, not something to worry about now.