Confused between my two cousins

[Arra]

I just experienced the scariest facial recognition problem I ever have in my life tonight. I went to my relatives' house for dinner. (I don't see them often, maybe once every couple months or so at family gatherings. The gatherings are pretty formal. The family is somewhat rich and they care a lot about how they look, what they wear, money, etc. So the atmosphere is pseudo-laid-back. I spend most of the time trying to talk to people, but being the antisocial person I am it's always difficult and stressful. Conversations don't last long and end in awkward pauses.) Two of my cousins, who are sisters, have a similar look. They're both pale redheads and are on the somewhat the pretty side. The younger one, about 12, let's call her Kayla, I was told early in the night had just gotten out of grade 7. The older one, about 18, let's call her Rachel, is in University. I was sitting, talking to others with Kayla for a few minutes. I was thinking about the difference between her and her sister, that Kayla is a little chubby, and I was thinking about how old she looks. We started talking and she asked me about school. I told her a few things, and then I decided to ask her about her school. I said that I heard she's just graduated grade 7. She smiled a bit and said "that's her", pointing to her sister, and then I realized, I'm talking to Rachel! I immediately apologized and tried to make excuses. I pretended that I had some mental malfunction and that I had no idea what I was thinking, that I don't know what was going on in my mind. I made it clear that I knew she was way out of grade 7. She was smiling but seemed okay, and kept saying it was okay as though she realized I felt bad. That might have been the most embarrassing thing that's ever happened to me. I just hope it wasn't too horrifying for her, and that my excuse was at least believable. How the fuck did I mistake my 18 year old cousin for her 13 year old sister? It seems there must be something very, very wrong with me. It was one slip-up, one of those strange things that happens in life, I hope. I just don't understand how it happened. It scares me. One possibility is that, for some reason, I created a mental image of Rachel in my mind, how I used to see her. Rachel has gotten slightly chubby, and that’s how Kayla has always been, while Rachel used to be quite skinny, and I guess I still have that image of her in my mind. I do have a hard time recognizing faces. I mean, if I meet someone once briefly, I probably won't recognize them when I meet them for a second time unless I intentionally paid close attention to their face. But this wasn't some brief encounter with a stranger, it was my cousin I've known for my entire life. Not seeing them very often shouldn't be an excuse. It's one of those things that, for the rest of my life, I'll be summoning an urge to punch myself every time I think about.