Pre booked holiday but starting new job

have recently been offered a position for a job I applied for. However 6 weeks after my start date I have a 17 day trip booked to go see my family abroad (which I have had booked for a year). I have made the employer aware of this and they said “don’t worry it won’t be a problem. We will manage.”

However, my dad is now saying that I look extremely unprofessional and that this will reflect badly on me and be taken into consideration upon the end of my probation period (which is 6 months). He is saying that it will annoy other employees if I go on holiday so early into the job and that is very risky and that I should cancel the trip and just go next year once I have the job secured.

What do you think? Should I rebook the holiday even though the employer said it’s fine? I just really don’t want it to reflect badly on me.