Dear N.,
Yep, your mother is right, and yep, you are doing the right thing, just keep offering them different foods. They must have had a 'conference' and decided to worry momma.
If they get hungry, then they will eat. I volunteer at the nearby Salvation Army every Friday afternoon. One lady came in and said that she was homeless - and told me why. She is now living with her son, and since she has been with him she has been eating a lot of food - 'like a pig' she said. When before she just never did eat and wasn't hungry. So now that her body is used to food, she is interested in eating again. She was pretty thin, and very happy with the food we were able to give to her.
No kidding, they will eat, it must just be a time in their lives when their bodies are consolidating or not growing much. Pretty soon their bodies will begin to make some changes and then they will eat.
We think that we are in control of our minds and our bodies, but we are not. "They" want to live and will bring about feelings that we recognize as hunger, so that "they", our brains and body, will survive. Good Luck, you are very lucky to have your mother around. Some mommies don't, and it is a very big burden for them. You know about the natural law, don't you? That Every Mother needs a Mother. True.
Little girls are so lovely, and you have two of them !! Sincerely, C. N.