So, my FI's sister is the youngest of three. She definitely acts like it. It is unbelievable to me how textbook she is; she acts like she's five, whines if she doesn't get her way, and complains openly when she feels like her older brothers aren't spending enough time with her/giving her enough attention. She doesn't say please or thank you hardly ever, and she truly will just whine instead of asking for things. Like when we sit down to dinner and she wants the water pitcher, she makes the most irritating sound, waving her glass in front of her. She is almost 20, and acts like a child. It's gotten increasingly worse the longer FI and I have been together (or maybe I just noticed it more).
That said, when she is not acting like a child (I wouldn't hesitate to say she is childish at least 30% of the time), she's a great girl. She's shy, but has awesome tastes in books and music and we really bonded when I first started dating her brother (the middle brother was dating a horrible woman, and I sort of became her antithesis). I've made her a bridesmaid (please don't give me a hard time about picking too early. I KNOW. But I made my mistake, and now i have to live with it), mostly as a gesture to the family, but also because, like I said, when she's not acting spoilt and whiny, she's awesome. However, this is not a BM worry. This is more a FSIL worry. I'm genuinely concerned that she is never going to grow up, and she's going to act like this until we're all old and gray.
The thing is, I've had people tell me that she probably is acting this way, and getting increasingly worse the longer FI and I are together, because she's always had her brothers' rapt attentions. They grew up on a farm, and only really had each other to hang out with when they were younger. FI says he and the rest of the family have been around her so long, they don't even notice. I've heard them laugh at her totally inappropriate behaviour before, and it absolutely shocks me.
I guess what i want to know is, am I alone? Are there any other brides who have to deal with FSILs who are the youngest, and really know how to act like it? What should I do? Should I even do anything?