My siblings believe my dad has dementia, others don't. What is the best strategy for my dad?

Diagnosis by neuro-psyc doctor.

You are going to have to give more details and background information for anyone to give you meaningful advice.

A quick answer might be to get a referral from PCP, if necessary, and take your father to see a geriatric physician and/or a neurologist. These doctors would be better trained in diagnosing dementia.

Who are the others?

Does your father live alone? Who has most to do with him from day to day?