B1/B2 once rejected. Applying again

[Vd5295]

Me and my girlfriend are preparing for usmle. I already have B1/B2 visa but hers was rejected in 2018. We wanted to apply again for her visa and i have a few options in mind. There is a graduation ceremony of my sister in june 2021 in US or i have my uncle who is a citizen of US who can send a sponsorship letter or we will have an exam which is only held in USA to apply for the visa. What is the best way to apply for the visa.
Shes not currently working.