"She can't keep the sercet to herself."點解？

the sentence is correct and the grammar is correct too.

that just means she (herself) can't 保守 that secret .she has to tell that 秘密 to someone.

that can be a negative meaning if you say this, meaning she is not able to 保守秘密.

The meaning is 她不能保守秘密. the sentence should be: She can't keep the secret.

她跟本沒法相信

’她不能對她自己保守秘密’

是不是她不可以對自己說謊.
我看到的意思是這樣-___-"