So My sister's mother in law goes as far as responding to the kids when hey say mom. What can she do then?

I live in the states and she lives back home in Venezuela, her husband does not support her and it is always on the grandmother's side. My sister has repeatedly talked directly to her mother-in-law, and she just answers:"well they are my grand kids and I should be involved in they're upbringing too"... She calls me crying at least once a week with a new story. The latest the her mother in law allows the 3 year old baby to go potty on the bathroom's floor. And even after my sister has repeatedly talked to her telling her she does not want this to happen, she keeps doing it. This is destroying my sister and her marriage...any advice?