Step parent not allowed to parent

[Ohdearinthedoghouseagain]

First time poster here so please be kind.
Not sure if I’m after advice or just reassurance that I’m not just a horrible person.
So it happens I’m in the dog house again with my DH.
Mum of 2, and step parent to 1 here. I have been present in there life since they were a toddler so for a long time now. DSC lives with us full time and is in his late teens, has a gf and drives so can come and go as they please.

My problem is and it always has been that my opinion never matters in anything concerning DSC, It’s like they can never do any wrong, and that I always have to keep the peace. God help me if I ever pull them up on something or say something they don’t like or disagree with.
This happened tonight whilst my DH was out, DSC went out after we had words and has now moaned via text to DH about me which in turn has caused me and DH to have words and get annoyed with one another. Now in bed feeling very miserable.

My main frustrations are that it’s fine for me to do all the other parenting stuff like cooking meals, laundry, tidying up after them, helping teach them to drive, and prior to them driving lifts to and from there mothers, lifts to and from college/work/gfs, all the usual patenting stuff.
Me and DH have 2 DC together and I feel that they all should be treated the same and I should be able parent them the same as I would our other DC.

Is this unreasonable?

YANBU. They can't have it both ways. As long as what you're saying is reasonable and fair then you should absolutely be allowed to pull the DC up if they're out of line.

If you can't parent DSC stop looking after him. He's close enough to or 18 by now, he can cook and do his own laundry. I know that sounds simplistic but if your DH is going to be such a wanker about it then his son is an adult so you no longer need to treat him like a child.