Are children of different wives mahram to each other, and can they inherit from one another?

Yes, both. Children of the same father but different mothers are paternal half-siblings (li-ab). They are mahram: they may not marry each other, may shake hands, and doing so does not break wudu according to the Shafi'i school. They can also inherit from one another when there is no heir who excludes them, such as a child, the father or a full brother. As a family, parents are encouraged to nurture close ties between the children of different wives from a young age so that they help one another as adults, especially after the father dies. See children in polygyny.