how do you deal with your in-laws?

My future mother-in-law acts like I can't take care of her son or grandkids. It drives me crazy. My fiancé says she says things she doesn't mean without thinking... she's mad at me because he granddaughter hangs all over me in front of her and won't spend time with her... I encourage my step daughter to spend time with her nana but she doesn't budge. We went over to her house over the weekend to help prep for winter, as we were making supper she made a snide comment "it's funny how my granddaughter is hanging out with me before you show up then all the sudden she wants nothing to do with me" I about burst into tears! I try my best to force my step daughter to spend time with her!! I told my fiancé how I felt and he said she likes you she just finds it weird that she won't have anything to do with her when you are around! I told him to either stand up for me and explain to his mom that I try to shake my step daughter off so she will hang out with her. My fiancé told me I'm overreacting but I feel I'm not. My feelings are hurt by how she said things. There are other similar situations in the past as well. Last month I was accused of coming between my fiancé and his daughter over my daughter and my step daughter throwing grass at me and down my shirt so I started to play back and my daughter threw grass at his daughter and she ran into nana telling nana we were bullying her and not playing with her when we were. His mom sent nasty text messages to my fiancé and I accidentally read them while he opened and read them himself. I just want her to realize (nana) it's not easy blending a family and we are trying... it feels like she's judging me! I bought all 3 kids clothes and sacrificed getting myself winter clothes.