How long before my ex and her boyfriend breakup for a 3rd time?

My now ex girlfriend left me, mostly because her 26 year old daughter which was staying with us, had a falling out. They, and the 8 year old grandson, which was also staying with us, went to go stay with her ex. I discovered in this time frame that she had been talking to him about our problems and they both were saying via facebook I love you and I miss you. When I confronted her about it, she said she was just telling him what he wants to hear, so she could get her daughter and grandson to stay til they found their own place. I know they're back together again, and I feel it's more due to convenience than actually being so in love. If that were the case wouldn't she have left me for him before now? The daughter is getting her own place next month, but my ex is going to be staying with him still. I would like to try to work it out with her, that's why I'm asking this question.