I have interest income from an account that I share with my Mother as a joint holder from a foreign country I am a citizen of (My mother is both a citizen and a resident of the foreign country). The account states her as holder number 1 and me as number 2 on the certificate. I currently file taxes as a resident in USA.

The account was funded by me for the purposes of my Mothers upkeep. All the interest derived from the account (Equivalent to USA CID) is transferred directly to my Mother's savings account in the foreign country. My mother is currently 57 years, single parent and with no job files a certain tax form that relegates her from any interest in the foreign country.

Would I need to declare and pay taxes on this income, if so would it be the full interest income or a percentage of it. And do I need to add any other documentation providing explanation / proof.

Also what part of the principal would I need to disclose in the Fbar etc.

Appreciate any help in this regard.

Joint account is funded by you so all the money is yours.
If you fund your mother's saving account, you may treat the amount funded by you as gift. Then all the interest in her saving account is her income.
On FBAR you must report the account where you have the signature authority.