Your story sounds so familiar.
My fiances brother and his wife are having a baby about 10 mths before our wedding. He has said that they cant go, which really dissappointed him..he is the best man. He actually, as of now, changed his maind and said he is coming by himself, his wife and the baby will stay home.
His mother is also treats her 3 sons completely unequally, at the low end of the stick is my fiance. Both the other brothers are married. Got generous gifts and there is no gift in our future.
My mother passed away. I think this tops it all. It is the worst way to plan a wedding...without your mom. I have been leaning on my best friend (best friend for 18 yrs) and my sister. My future mother in law isnt the easiest person to deal with and isnt exactly the person that I want to try to lean on through the wedding process.
I know how hard it is to stay positive. All I think of is how this is supposed to be the best time of my life. It is never going to happen again. Its definitely hard.
Try to strong and think of what your mother would want you to have and feel. That is what is getting me through it. Whatever happens with your in laws, there is nothing that can change it. Just go with what happens. That is what I have come to accept.
Good luck!