Every year

Every year, my sister-in-law L tries to plan a Thanksgiving dinner for the whole family (in Canada, so the first Monday in October), and every year, my other sister-in-law R plans her own dinner for just her husband and kids. Every year, L gets really upset and sometimes even cries because she feels excluded. It's usually just L, me, and my husband for dinner. I feel bad for L, but I also think she needs to stop getting her hopes up. It never has worked, so why does she think it will?