Can anyone who speaks viet translate this for me?

That's what I thought at first too. I took it literally

sinh - ask for permission
minh - myself

ask myself for permission = allow myself. Usually in Viet, that means "asking my mother if I can go hang out".

Yeah, that's what I thought. Problem here is, then if that's the case, all of the 'me's used in the letter would indicate that she was referring to her mom? Strange.