Am I allowed to be informal?

My family's maid Lupita comes every Friday... something that has been routine for almost 8 years or so. I live in the Southern part of Texas where people speak Spanish just as much as English, so I speak it pretty well. The thing is, when I communicate with her, I use "usted" and its verb conjunctions with her while she uses "t�" and its conjugations with me. She's a woman that looks to me to be in her late 50's while I'm a 16 year old boy. Am I allowed to use "t�" with her, or should I keep using "usted?" Thanks!