My husbands ex is trying to get the kids back, can she?

Ok, so my Husband has custody of his 2 sons. His ex wife signed custody over to him 2 years ago. She lives in Texas and we live in Hawaii because my husband is in the Army. She is trying to get custody back, how likely is it that she will get it back? The boys are currently visiting her for the summer and apparently she has convinced them to want to stay. So she called my husband and asked if they could stay (basically asking to sign them back to her). She said she is getting a lawyer but we have legal custody paperwork from 2 years ago. But she gave up her rights as custodial parent, she can't just take it back when its convenient for her. We have already contacted our lawyer in Texas but since we live in Hawaii the time are completely different and he hasn't gotten back to us. I need to know it she has a chance. If you have experience in anything close to this please respond.