Every mother dreams that her son will marry a nice, loving girl who will embrace his family and easily fit right in. Many times this does indeed happen, but other times the exact opposite occurs. Some daughter-in-laws are manipulative, controlling and selfish. These women are very difficult to be around, and can cause a serious, sometimes irreparable, strain on a family. As her in-laws, you and your spouse must tread this matter very carefully in order to avoid this woman turning your son, and any grandchildren that you have, against you.
Start your relationship off with your daughter-in-law on a positive note, even if you really do not like her from the start. It's more difficult to turn a bad relationship with her around than it is to just force yourself be nice to her, and try to win her over.
Go along with her demands, as much as you can. This probably seems incredibly undesirable, but unfortunately it is the best way to go. Keep her happy so that she has no reason to turn against you.
Talk to your son if her demands become too outrageous. For example, if she refuses to let you see your grandchildren, or will not attend family functions, this is a serious matter that needs attention. Do your best to speak highly your daughter-in-law to your son, because if you are full of negative comments about his wife, no matter how true they might be, he might take her side.
Seek help from a counsellor, as a last-resort option. Family issues, especially related to your children, can be very stressful and consuming. You might find yourself experiencing depression if your manipulative daughter-in-law has turned your son against you, or will not allow you to spend time with your grandchildren. A professional can help you come to terms with this, so that these issues don't take control of your life.