Hey skinnick! I'm glad that your family member told you about this discount and that you enrolled now, because the idea of you guys hanging out over the holidays and comparing uneven bills instead of drinking hot cocoa bums me out! Customers on our T-Mobile ONE plan do get a discount for enrolling in AutoPay - $5 per line, so if your family member has three lines, then their bill would definitely drop $15. I do have to say that this discount doesn't apply the first month, though - you'll get a double credit the second month that you have both T-Mobile ONE and AutoPay active.
As for other reasons that your bills are uneven - I can't say for certain since we don't have account access here, but I can try to give you some possibilities. You may have a higher monthly statement because of equipment and/or accessories costs you're financing, your data plan selections, or insurance for your line(s). I hope that helps point you in the right direction, and that you enjoy getting lower bills soon! If you have any other questions let me know!