My MIL was awful for a long time. She said she had 1 grandchild (when I had 4 children), she barely acknowleged my other children. She did buy them all gifts on their birthdays and Christmas, but it never was quite as "loving" as the things she got for "her grandbaby". My dh finally had a talk with her about it and told her that he had 4 children, that if she could not accept them all that she would see none. Now, wile I know in my heart that she doesn't have the same feelings for my first 3 that she has for my (now) younger 2, she does treat them all with the same respect, and I guess that's all that I can ask for.
However, my oldest dd's dad remarried a few years ago. They have 2 children (well, technically the older one is theirs, the younger one is hers with another man...there was a long seperation before they were married...long story). We spend Christmas', Thanksgivings, Easters, birthdays, etc together. We babysit for each other, we buy each others kids gifts, we are all one big happy family. We love each other, we fight with each other, we hang out with each other, we have barbeques together. We have never once considered that all of our children are not related. Once, it was suggested by someone that my ex and I still wanted to be together. We balked as if sleeping together would be the most absurd thing ever...even though, obviously it's happened, lol. We're just such good friends and he is just like another family member at this point.