My two sister and I are co-trustee of my mothers trust.My oldest sister has been taking care of trust until my mother past away.

Your attorney is right.  If your oldest sister won't cooperate or settle, the only thing you can do to "force" her is take her to court.

In an ideal world, people who disagree would each go and talk to their lawyers, giving completely honest and accurate versions of the facts, both sides would get the same answers and, whether through counsel or directly, settle the case pretty much where a court would call it, without the delay, inconvenience and expense.

But people see things differently, and there are legal questions where there can be honest disagreements as well.  We have courts to settle differences, when the people involved can't do it themselves.