Finally confronted my siblings about taking large amounts of money from our elderly parent with dementia! What can I do?

In January you posted about possibly hiring a fulltime caregiver? Has this happened?

What is your parents' financial situation? What is going to happen if your siblings manage to get so much of your parents' money that your parents can't afford caregiving help, a facility, or become Medicaid-eligible because of all of the gifting?

Are YOU going to be the default caregiver if this happens? Or are you going to be the one who hires and pays for caregivers or even facility placement?

Your sibling is correct. If the parent is competent and managing his or her own finances he is free to give any amount of money to any child he wishes to, or in fact to anyone else. Quite honestly it is not your business.
If you believe that a parent is demented and is in danger of giving away their own money, unable to manage their funds, in danger of gifting and hence not eligible for medicaid in the next 5 years that is indeed something to worry over. But that requires a diagnosis of dementia, the appointment of a conservator or guardian, and etc. You don't mention that any of that figures in this.
If your parent is diagnosed with dementia let us know. Different answers for different situations.

Yes, what they say is all true. However, the combined effect of it all may not be what your Elderly Parent has ever intended. Perhaps a good option is to take EP for a meeting with a lawyer/ financial advisor, to consider what EP wants as the end result. If EP can make a preferred ‘final solution’ disposition, with some discretionary ‘spending money’ available now, everyone’s best interests might be protected.