Hattiehelga, AmMaz, I am sorry you have had such sad problems with your DiL, but because your experience has been unhappy doesn't mean that is everyone's experience.
I married an only son and had the happiest and most loving relationship with his mother. Sometimes better than the one I had with my own DM. My relationship with DDiL is exactly the same, we enjoy doing things together, swap clothes, and I cannot believe how lucky DS has been to get such a lovely girl to marry him. Both families are quite small so have bonded together to become one. When we turned up for the 80th birthday of DDiL's mother, she thanked us for coming (she lives 200 miles away) because it was so nice to have all the family present.
I know not everyone is as fortunate as me, but between your unhappy experience and my very happy experience there runs a whole gamut of relationships.
I think far too much is talked about MiL/DiL relations. It is no different to going to work and having to work with a range of work colleagues, some of which you like, others you have nothing in common with and still others you actively dislike. If you are to do your job successfully you learn to cope with all three groups and try to keep relations civil between you. Sometimes it will work, sometimes it won't. End of story. Relations with a MiL/Dil are no different.