so will eating a well "unhealthy" breakfast one day out of the week hurt me?

I think it sounds great, except for the Dr. Pepper but that's just me :-)

I do the same thing on Sunday's because the other days are oatmeal, smoothies, etc.

5'8" and 155 is perfect.

Many people who are very "into" fitness have one day per week as a cheat. And yours isn't that much of a cheat. It's balanced and it's not like you're eating candy and pizza all day.

Enjoy and ignore your sister on this one.

No, what you're doing is fine. I eat clean 6 days out of the week and have one set day each week to have myself a cheat meal. It won't screw up your fitness goals. In fact, a scheduled cheat meal will help you as far as keeping your self discipline. What helps for me is I plan what I'm going to eat that day so I have something to look forward to. Also, I recommend having it earlier in the day so your body will have and ample amount of time to burn it off.

In addition to that, I also allow myself two days to drink alcohol each week, but I never drink alcohol the day I have a cheat meal so my metabolism is in good shape to burn it off. My drinking days are also separated from the days I lift weights so I avoid drying out my muscles and causing muscle catabolism.

So no, you are doing just fine. As long as you stick to your goals and meal plans for the other 6 days, you are on the right track.

Nope.

Just exercise.

You'll burn it off.

No, it's good for you :)