M.B.
This same thing is going on with my aunt and the best advice I can give you is tell your daughter when her boyfriends kids are away for the weekend she needs to spend 1 on 1 time with him. Im sure thats all he needs. Does she work? Im guessing thats were she is when you watch the little one. Maybe she can ask for 1 extra day a week off or maybe just get out early every once and awhile. Its hard going from getting all mommy's attention to having 3 more people and three more dogs all getting his moms attention. She could take him out just the two of them... maybe the park, a movie, something were they'll both have fun and hell know mommy will always have time for him.