Time off of new job?

Hi all, new here so I'm not sure if this is the right place to post this.

I was recently offered a new job, but while working at my current job, I was approved for time off towards the end of August, so I bought the tickets and everything. Now, I start my new job in a few weeks, and I'm not sure how I should ask for time off. The new job comes with benefits such as PTO, but I won't be eligible for that until one month of hire. I NEED this time off because I'm helping my sibling move into college, and everyone else in my family will be too busy to help.

How can I ask for time off without looking bad? I'm thinking about emailing my boss asking to talk on the phone BEFORE my start date. Any help is appreciated.

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