Despite repeated reminders sent between 1 PM and 7 PM, the individual remained absent when the guests arrived and dinner was served around 8:24 PM. After the meal began, this person eventually came downstairs upset that everyone had started eating without them, leading the OP to question whether they were in the wrong for proceeding.

1 PM: We warn her guests will arrive at 7 and to please get ready and be out of her room.
1-7 PM: We send sporadic reminders, knock on her door, remind her she has X amount of hours left, constant checking-in.
7 PM-8 PM: Guests arrive. She is nowhere to be seen. Everyone is busy doing something. Cooking, setting the table, socializing. Still we send reminders: People are here. Eating soon.
Dinner is almost ready. Dinner is on the table now. People are on the table now. We are eating. She is not answering or otherwise communicating with us.
8:24 PM (timestamp from photo): We start eating.
10 mins later she comes down pissed we started without her, nearly an hour and a half after we told her to be ready, and with constant reminders and warnings leading up to dinner.
Mind you there were 11 people total, with two sides of the family seeing each other for the first time in years, plus a newborn baby who was mercifully asleep right when the food was coming out hot.
Everything was chaotic and people were hungry.
Conclusion
The central conflict revolves around the clash between the logistical necessities of hosting a large, time-sensitive event and the individual’s apparent disregard for agreed-upon schedules and multiple warnings. The OP feels justified in prioritizing the needs and hunger of eleven other guests, especially given the circumstances involving family reunion and a baby, while the other party appears upset about being excluded from the start of the meal.
Given the extensive warnings provided versus the individual’s failure to appear on time for a major event, was the OP justified in starting dinner without them, or did initiating the meal cause an unnecessary escalation of conflict? The core question is where the responsibility lies for the breakdown in coordination.
Here’s how people reacted:
Are your parents still with you? Sorry i had to ask coz OP did not mention anything about your parents being in the same house.
How old is your sister?
Who eats thanksgiving dinner at 830? By 830 I’m home and showered getting ready to sleep off the food coma.
This shouldn’t even be a question, she’s an entitled princess and no one should be putting up with that nonsense.
NTA
That’s too late, I don’t think anyone is the assshole.
NTA