Dear future brother-in-law,
Welcome (almost) to the family! Before the wedding, I would like to lay down some ground rules.
First of all, she's my best friend, and it's going to stay that way. No matter how long you and she have been together, our 18-year-old bond has existed before you, and will continue to grow no matter how long you're on this Earth. I'm sure I'll be calling at 3 a.m. when you two are snug in bed because I just went through a break-up and I need her to calm me down, and a dinner date with me is probably going to take priority over dinner with you. Just because you're in her life now doesn't mean I'm out.
Secondly, you bet we're going to have sleepovers at your house. When she and I are four bottles of wine and three Disney movies into the night, it's more likely than not that I'll be crashing at your place. Your kitchen might be littered with half-finished bags of microwave popcorn and empty boxes of watermelon Sour Patch gummies, but I ask that you don't get too upset. I'm sure I'll just be jealous that you get to see my sister every day like I used to when we were kids.
Also, I will spoil your children whether you like it or not. I've already planned on being the "cool aunt," and there's nothing you can do to stop me. The only people I may end up loving as much as I love my sister are her little ones. So trust me when I say that loads of presents and unnecessary toys that have flashing lights and loud noises are on their way already. In return, I promise that I will be your go-to babysitter. You can call me any day of the week for any reason, but just know that those organic, taste-free snacks you will tell me to give them just won't do.
Lastly, you will be my brother. Now I don't want to get all mushy here, but it has to happen. With you marrying my older sister, you're legally my brother-in-law, but I don't want our relationship to be strictly lawful. I will look to you for advice almost just as much as I look to my sister. I want to be able to come to you when she and I are fighting because you're the only other person in the world who knows how stubborn she is. I also want to be in your life as much as she is in mine. I know I may seem like the annoying little sister of your girlfriend who makes too many inappropriate jokes at the dinner table to make you uncomfortable, but if you are as serious as you say you are about marrying my sister, I'll learn how to be serious too. I mean, I've always wanted an older brother.
Love,
Your future sister-in-law
(P.S. Good luck)
