How can I bypass Security Lockout on iPhone 15 Pro Max without data loss?

Sorry, but if the passcode has been forgotten, the ONLY option you have is to erase the phone using a computer with either Finder (Mac) or iTunes (Windows). Assuming your sister had been backing up the phone, she can restore the phone from the most recent backup. If she hasn't, the data on the phone will be gone and there is no way to avert that.

Follow the directions in this link --> If you forgot your iPhone passcode or your iPhone is disabled - Apple Support

This is why backing up ones data is so important before catastrophe strikes.

You aren’t speaking to Apple Support here, we are rather Apple Volunteers on this user to user technical support community. But even so, Apple Support cannot help you to unlock your iPhone. Your passcode is encrypted and stays on your device, meaning only you have access to it and can reset it. But by resetting it, your data will be wiped and the only way to recover it is with a backup. Otherwise, your data will be gone. As a result, follow this Support Article below.

If you forgot your iPhone passcode or your iPhone is disabled - Apple Support

Ahmed21Elbahy wrote:

Is there any solution that would allow us to unlock the device and keep all the data intact?  

No. anything that was not already backed up or synced to the cloud is gone.

Your sister can check to see what data she has synchronized to iCloud by logging into https://www.icloud.com/ .

Note that if she was not synchronizing data to iCloud, it's too late to save any data on the phone by turning that on now. You must enable synchronization on each participating device – and it sounds as though she won't be able to change any settings on her phone until after it is reset and wiped.