Read other letters about this article
Assuming the LW's husband will support her actions, I training the mother-in-law the same way you would train a puppy. Every time my golden retriever puppy jumped on me or my spouse, she got kneed and fell flat on her back. It took her two days to learn she should not jump on us.
If the mother-in-law misbehaves, punish her. If she comes over unannouced, don't let her in. If she refuses to hand the child back, go and take it, and then ask her and her husband to leave your house. Instead of having dinner at your or her house, meet them somewhere public for dinner. If she cries at dinner, leave the restaurant. She will quickly learn.
SJ