I have been "blessed" with two - yes two - mother in laws. Hubby's step mom isn't a problem, although she seems to think that we are not related to her since she just married his dad. We get along well enough and i usually enjoy time spent over with her and her son and his family.
Now, his biological mom is a different story. I have never been good enough for her boy, she lets me know that every time we are together, and i dread every single moment i am forced to spend with her. I usually come away at best with a headache and aching jaw from grinding my teeth in an effort to keep my trap shut. Sometimes that is a losing proposition. On those occasions i come away with a husband who thinks i was completely right but should have kept quiet and preserved the peace.
You very much CAN choose people you wish to include in your family, and I have on many occasions throughout my life. I haven't however found a way to separate myself sufficiently from the ones mother nature chose to stick me with!