My sister just fainted outta nowhere and took her about 3 minutes to "come to." i keep telling her to go to the er, what's the chance they'll admit he?

I depends on the: clinical circumstances, the history obtained and the physical findings and laboratory findings. Fainting is not normal so she should be evaluated regardless of the concern about admission or not. This can always be discussed with the ER health care provider, it is not like being taken away by the police and put in jail. She will have a choice and options.