I talked to my son once about the advantages of having a parenting certain routine (nutritious meals for kids, a bed time, potty training, etc.) but he told me that they are against routine and discipline and that their toddler «will potty-train herself "when ready"» and «wean herself "when ready"» etc.
As you are obviously aware, it is the parents who
have custody and control over your granddaughter.
There is no way that you can influence her, unless
you develop a positive relation to her parents,
so that they choose to let you take care of the
child again. They have to feel that you are not
counteracting their wishes.
What is happening now may also be a kind of triumph for
them: Mother, you decided over me, when I was
a child, but now it is we who decide!
You need not be worried about potty-training.
She will learn, whether she is trained or not.
To sleep in her parents' bed need not be a
problem. It can give her security, especially
since she is in day care during the days.
Children can be influenced by the people around them,
even if they do not meet so often. Try to get
your son's family to accept and like you.
Children who are brought up with order and
discipline will more often become
conscientious and well-adapted adults.
Children brought up with a great deal of freedom will more
often become creative adults. Society needs
both kinds of people. Some children, who are
brought up in chaotic circumstances, will
still do well later in life. Often a reason
for this is that a grandparent or a neighbour
has influenced them.
I know of people who have been brought up in
the way you describe, and who have chosen to
live an orderly life. Not because they have been
trained that way, but because they decide themselves
that this is what they want.