First consideration is- how much are the cemetery plots worth? That's going to be the deciding factor.
Because it spans two generations, and multiple parties, you would probably have to probate both mom and grandma's estates, UNLESS you could get grandma's other two kids (and their kids) to agree on a distribution- and then proceed under your state's "small estate" (affidavit) procedure. And I can just about guarantee that at least one of your cousins won't agree to your family getting the proceeds. Even if everyone thought, for the last 60 years, that the plots had been deeded to your mom, their tune will change when they find out that now they have a chance to get a few bucks.
This would be worth a 50 or 75 buck conference with someone who actually knows what he is doing- that being a probate lawyer.
As far as the rest of your mother's assets, I don't see any reason to do a probate. BUT if she had any debts, you may need to do a Non-Probate Notice to Creditors. Ask the attorney when you talk to him.