Feeling timid/shy/scared and acting that way can be very different things! When we are scared or insecure we have to "control" our environment and that means the people in it. My niece did that; when she was 1-2 would stand at the full-length windows inside the house and "yell at"/boss the kids playing outside down in the circle/street! She couldn't even talk yet so we adults found it funny as she was clearing bossing them around, waving her fists, telling them what to do but this was from inside the house, window closed, they too far away to hear her.

I wouldn't worry about "forgetting" your SIL either; if you liked her and then she "betrayed" you/your brother by divorce then that is painful! It is so much easier to remember things one way, all good or all bad then to realize that life isn't neat like that and there is always both happening at once. What do you remember of that holiday? If you don't remember much at all, then that you don't remember your SIL much is just part of that and/or if something more "exciting" happened with your brother/s that would take your memory away from less exciting/normal occurrences. We remember what is striking much more easily than the "good"/even-tempered stuff.