It sounds like the stresses and strains of work/life balance for your daughter. With 2 little ones and a job it sounds as if your daughter is struggling to find time for everything. She will be ignoring her own needs and as a result, being impatient and snappy. As you say, you don't want to interfere, but instead, is there anything you can do to help out?
Could you meet your GD from school a couple of times a week and give her some one to one time before tea? Have both children for half a day at the weekend to let your daughter have some time to herself? Etc