Her unmarried children can accompany her to the US as long as they are listed on the fiance visa. In addition they must be under the age of 21 "and" unmarried when the visa was applied for.
Regardless if the daughter is in the US or not... you will most likely be sending money every month to support the daughter if she stays behind. Think about it, you take her mother and (most likely) the daughters only source of support, that makes you morally responsible. Besides that the mother will not allow her children to stay behind without helping them.