LOL.... oh my.
My MIL and I are very, VERY different people - we have learned to accept this and we get along. I appreciate who she is and all she does for our family - I always stress to my DD that she treat her with the utmost love and respect and they do share a special bond. I encourage that.
My BIL and SIL are a different story. DH's brother and his wife act uppity. My DH made some mistakes when he was young, as do most people, and they seemed to label him a loser. When I came into the picture, SIL made it clear they didnt take the relationship seriously. When she realized I wasnt going anywhere and that DH and I seemed to improve each other's lives, she felt threatened.
Her DD was 5 yrs old when I started dating DH. I have never known that girl to not be on a diet. SIL would tell her "No, you dont need that." If DD asked for seconds and dessert??? Forget it! And my DD is tiny (DH and I are short so this is expected.) SIL is always saying, "OH, Kristen was so much smaller at that age. At 5 years old, she wore a size 3." As if she were extremely proud. My MIL speaks sometimes without really thinking. One day she saw Kristen eating and said, "Oh my, she's eating lately." (The girl is 13 yrs old, of course, she's gonna eat a bit more- she's growing!!) and my SIL halted all conversation and said, "OH NO, she has to watch what she eats now. She's in cheerleading. SHe really has to watch."
At this point I'd had enough and said, "A 13 yr old has to watch what she eats? You dont find that disturbing?" But i'm talking to a wall.
The girl is really mean to my 6 yr old DD. But now, my DD just looks at her and says calmly, "Kristen, does that make you feel good? Talking to a six year old like that? Do you feel big now?"
SIL and BIL are always trying to compete with us. When we got a big screen tv, they found out it was a few inches bigger than theirs (they asked how big it was, I had no idea how big their tv was, nor did I care!) and SIL said, "Yea, well, we needed a new tv anyway." ANd honest to goodness, they went out the next day and bought one bigger.

When i went back to college, SIL had to go get her beauty license. I attended her graduation and got her a gift, etc. When I graduated, she refused to come, and would not even look at me when I mentioned getting my degree.
When I got my job, my MIL was telling her about it.... and she hung up. And wouldnt talk to me for months.
Now, DH got a new job recently. They are livid. I dont understand this. They do well. She inherited money and land. Why do they feel so threatened by all this? I dont get it. My BIL even said to him that he would never work for that company, you have to bust your a$$. My DH is the type that has to be working hard or the day just drags for him. Then BIL said "OH, that's why youre making what you make. You have to work in the freezer. A lot of guys I know cant handle that. " I finally spoke up with a smile and said, "Matt is more of a man than most." LOL I said it in a light, joking way but it didnt go over well. If DH tried to talk about the job, such as when MIL asked him about it, they changed the subject quick and wouldnt let him get a word in edgewise.
It used to bother me, then I found it funny. Now I find it sad and i just dont understand it.