My twin sister is able to unlock my iphone with her face is there anything i can do about it

See this, About Face ID advanced technology - Apple Support You can try deleting your current Face ID and try setting up a new one. Make sure you are alone when you do this. The other option is to disable Face ID and just use the passcode.

har2929 Said:

"My twin sister is able to unlock my iphone with her face is there anything i can do about it: do the newer iphones help so this can not happen"

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Use a Mask:

Put on a mask, and always wear the mask (I guess?).

Use the passcode.

You’re in the too-closely-related-to-tell-apart range.

The probability that a random person in the population could look at your iPhone or iPad Pro and unlock it using Face ID is approximately 1 in 1,000,000 with a single enrolled appearance. As an additional protection, Face ID allows only five unsuccessful match attempts before a passcode is required. The statistical probability is different for twins and siblings that look like you and among children under the age of 13, because their distinct facial features may not have fully developed. If you're concerned about this, we recommend using a passcode to authenticate.