Forgot My Sister's Birthday

So yesterday was my sister Lily's birthday and I totally forgot. I mean, she’s only like two years older than me but it feels like forever since we were little and she'd make me do all this stuff to ‘celebrate’ her special day. Anyways, when Mom mentioned it at dinner last night I just froze. She asked if I got Lily a card or something and I had nothing. I felt so bad but also kinda relieved? What’s up with that? Now I’m trying to figure out what to do for the rest of the week until her birthday comes around again next year. Maybe buy her chocolate or flowers or whatever, hoping it can make up for forgetting entirely.