I think as long as the same rules and consequences apply to every child, then whatever the outcome is, is up to them.
Here's an example of what I mean, my parents said as long as we maintained a B average in high school they would pay for any college of our choice, that we could get into, for 4 years. My older brother didn't apply himself, didn't get into a good college, and went to community college. I did apply myself, got into a great school out of state and my parents agreed to pay. When my brother tried to throw it in their faces that HE was slighted and that they didn't pay to send him off to school, my dad reminded him that the same rules had applied to both of us. The fact that I went off to school was because I took advantage of the offer.
I think my mother always felt bad for my brother. She felt that he was slow to mature and needed more time to get his act together. So the "rules" weren't fair because he couldn't live up to the deal that was made.
I learned a lot from that experience. I think that in order to keep things "fair" then kids need rules, limits and consequences laid out, in advance.
I work hard with my kids to ensure that things are "fair". Not based on what one kid does, or another doesn't. But based on "house rules" that everyone has to follow. My kids understand that there are "logic consequences" for not following the rules - like chores, doing what they are asked, etc.
I don't think it's unusual to have some minor differences in the "consequences" based on the child. So that if one gets the computer privileges taken away and the other gets something else (one thing) then that's fair. I could see why it would be viewed as unfair if it's all electronics for one and not the other.
In our house, my brother clearly got in more trouble than I did. However, I think that only escalated the level of punishments he got. So maybe the first offense would be 1 electronic, then if he earned it back, the next offense would be 1 electronic. If he had not yet earned it back and had another offense, then it was 1 more electronic, so 2 total. Then if I did something wrong, I'd lose 1 electronic. He'd complain it wasn't fair because he'd lost 2 things...however my parents would remind him that he had two offenses.
Try to write out some of your "logical consequences" so that the punishments fit the crime. They should be built upon so that each offense builds with a steeper punishment.
Although it sounds like Chey would never get past offense #2 (cumulatively), maybe Chels would. However she would know that SHE "earned" her level of consequences. And then you can get out of feeling guilty and you can always refer back to your list of "consequences" to remind her of how she got to her level of punishment based on her past behaviors.
I hope this helps - sorry it's so long.