I have a question about my taxes

My husband and I have full legal guardianship of our grand daughter through the courts in my county. Our son died and our daughter in law was proven unfit after his death. Our grand daughter has been in our custody and home since 9/2015 and we have provided all financial support, the only exception is her death benefits (social security) from her father (our son). My husband and I told our daughter in law that we would be claiming Rosalyn as a dependent  for the 2017 year. My daughter in law has claimed her every year even though she has not provided one cent of monetary support. What do we do if the daughter in law files first?