C.O.
G.:
I take it your SIL moved back "home" while her husband was deployed?
Stress affects everyone differently. They could be adapting to life without Daddy. Or they could be (if they moved) adapting to a new environment.
Your sister is most likely stressed and while she is most likely NOT taking it out on the children - they are picking up on her stress and reacting to it.
Every child potty trains at different times! My daughter was 4 when she was FINALLY potty trained. My oldest son? Potty trained in ONE WEEK!!! And he had just turned 2 (and I mean JUST!!) My youngest son? Took him almost six months to potty train and he was almost 4 by the time he was potty trained.
If I were your SIL I would give my daughter PULL-UPS and stop pushing it. I know our society has these "rules" about "when" a child SHOULD be Potty Trained - it doesn't apply in all children. So tell her to drop it. Give her pull-ups and do NOT stress over it. Leave it be. She will get there.
I would strongly suggest that they skype their daddy so they can see him and talk to him. that will help. However, do I think his deployment is the cause of this? No. I think it's just life in general and the stresses of life.