I agree with raindelay. I spent some time at my moms as well after a break up, and my girls had to sleep in a bed with me because there was no room. As far as I am concerned, it wont hurt them a bit. It happened last summer with me, and my girls are fine. They have thier own rooms again and we are in a nice place. They sleep in their rooms just fine. There is no additional conflict since they slept with me. It may actually be healthy for them all since they are dealing with something so traumatic in thier lives. Give it time. They will heal and they will be okay.
Probably not going to hurt them. She and her children are dealing with the scars of a recent separation. She may need them there as much as they need her. We also don't know what they may have been through. At some point the mother needs to return them to some independence by having them sleep in their own rooms or at least a shared room. Plus, mom may want to get back to some semblance of an adult life at some point.
I wouldn't stress, they're trying to heal some hurt. Give it time.