OK, we've established that there are two women living in the same house that love the same man and child.
Their roles are a little different, but essentially the same - they love a beautiful baby. They also love a man, and want rights to him - you have new rights to him as your partner and baby's father. She wants to keep her rights as a mother. This is a type of natural rivalry that will not go away, never..
She will eventually see that she did not loose him, and that you are good for her son (most mothers have problems in this area - they want the 'best' and don't really know what that is.. ) She will continue to 'test' you to see if you are 'worth' it.
You will eventually realize that you are the new woman in his life, but that he will also keep the 'old' one and love you each in a different way.
Don't make him choose.
As far as her treatment of others, accept it as they have - they obviously know her attitude and their roles in her life - and even if this bothers you, just remember that you don't have to be in this forever.
It also gives you a chance to really get to know her. As was suggested, zero in on her qualities, compliment her on them, and make her feel appreciated - that always gets them on a better side. If she's not hurting the baby with her 'superstitious' ways, then let her - she can't be changed, but you can keep your cool and gain full control again when you have your own place.
We've all gone through this with the mothers of our men, and it does make us stronger and definitely more diplomatic. It helps you prepare for when your child is grown and starts bringing other people into your home - will teach you tolerance too.
If things bother you so much, write them down on posts to us, we can reassure you that there eventually is a "life after the inlaws"...
Good luck, and keep us posted.

Remember "when in Rome", and do your thing when you have your own "Home Sweat Home"