[Landlord-US-MD} Mother skimming money off rent, what should I do.

So.. I've been renting to a family for about 7 years. At first it was grandma, her son and his wife and their two kids. The grandma always paid the $1400 rent out of deceased husband's Social Security and pension. I never had much of a problem. Her son died a few years ago. He was a deadbeat with his wife so it didn't matter. Then grandma started getting sick and his son's daughter started paying the rent sometimes out of grandma's money. Every time I collected it from her it was always short and I just let it go. Eventually grandma died and now the house is occupied by mom, the two kids and two grand kids and still every time I collect the rent from mom it's short. She doesn't work like her deadbeat deceased husband. She cleans houses and babysits. I feel her kids are giving her the money and she skimming of the rent money. This month she called and said she only had 1250, them it became 1150 and when I pick it up 1110 and she acted suprised that it wasn't 1150. How would you handle this situation?