Noooo, don't do it! That's his lecture to give them, not yours.  That will push them away completely. He's a big boy, if he wants to let his kids know he's hurt, he will do it himself.

You are new to their lives, don't make that mistake.

Honestly no good will come of you texting them. Like you said, they're adults, and scolding them is just going to make your relationship - whatever it is right now - even worse. Let your husband say something, if he wants. Like a lot of divorced men, he will most likely stick his head in the sand and say nothing, and that's his choice, but let him deal with his children himself. You said you've only been married 18 months, so a scolding text to these "kids" is just going to alienate you completely. Trust me. 

Just disengage. You're not really a step mom because they're all grown. I'm not trying to diminish your position but it itsn't the same as if you'd been with them growing up. Just be their father's wife and live your life with him.  Be cordial, and don't get emotionally involved so you don't get hurt. 

is it appropriate for me to scold them for being so hurtful? 

NO. These are grown adults who have the right to invite whoever they want to their events. It's ridiculous that they excluded one of the most important people in their lives but that was their choice. If your husband is hurt then he needs to approach the subject with them and leave you completely out of it.  

i love my husband and i can't sit back and watch these kids use him.  he said he's used to it and it hurts but he takes the crumbs they throw at him. 

Then don't watch, because that is likely what is going to happen for the rest of his life.   He is used to their crumbs and resigned to the way things are.  How do you expect to change their dynamic and dysfunctional history, and why would you want to camp out in their train wreck???

I get that as his wife you want to protect him from his hurtful kids. You want them to change and be nice to their father. If you insert yourself into the first family drama they will get closer and bond, but it will be over hating you. They will all turn on you! I've been there, DH and his kids were so close when I was trying to *fix* things. I was seen as an interloper who was getting between them and everyone hated and resented me.  Take my hard earned advice and stay out of it.  

unfortunately pas’d out kids often lack those critical thinking skills, reality checks and logic or self reflection... they’ve been so brainwashed and dependend on the narcissistic alienating parent that this is all they know. They do not function effectively with their own opinions. Everything is run through the alienating parent...

I'm sorry you and DH have  joined the not invited parent/ stepparent group. Unfortunately, it's not a small group. It's membership waxes and wanes depending on the need of the bio parent to see the step and the openness of the bio parent's wallet.

Yes, it hurts to see them exclude their dad. But, there is nothing you can do about it except to support him in whatever he decides to do or not do.

Its best to say nothing. I hear you though, its very hard to watch DH and yourself being treated badly.  Its on DH that he just puts up with crumbs, thats his choice. Be thankful you arent invited to their toxic gatherings, where they can amp up their exclusion games . How nervy though, isnt it nice that SK1 offered up her house so YOU can throw ingrate#2 a baby shower. "Gee thanks, and yes I'm all over that".  These grown ahole adults never cease to amaze me!

I’ve been married to DH for 15 years. His kids were “adults” when we married. And still, just a few weeks ago, I really wanted to tell them how much their actions hurt their father. 

But I didn’t. And won’t. Rinse. Repeat. 

Enjoy being Dad’s Wife and stay out of their dysfunctional drama. 

My DH has a step-mother of a sort, she married his father after he was already a grown man.
A few years ago she sent both my DH and his brother a scathing text message about how little they do for their father and how much they hurt him.
DH was stunned at the text, considering he always goes to see his father, not the other way. He always calls his dad to chat, not the other way. He always sends cards and gifts. So it was a slap in the face from a person he barely knows to do this.
Needless to say it left both him and his brother feeling very upset with the stepmother because she wasn't their "parent" in any capacity and they felt if his father was truly upset with them, then why didn't he just reach out and talk to them?
Since that time we have gone to visit them (not the other way) several more times and she acts pleasant and polite and nobody has every really addressed the text.
Considering you are not a parent to them, let their father handle it. I understand being upset for him and wanting to defend him, but no good will come of it.

You just be as nice as.  Now you need to start saving money, because you and DH need to go on a really nice trip. Sun, beach, waves, tequila at sunset, coffee on the beach at sunrise, fishing poles, boats, swimsuits, and a very happy couple (that's you two) in Facebook posts. And, if you can afford it, treat a friend and their partner to the holiday as well. 
 

because the thought of "their inheritance" (and people like this think about their parents money as "their inheritance" all the time) being spent on strangers when they could have gone on the holiday instead will just grind their gears. And will remind them that you and DH have control of purse strings they have probably always thought of as theirs. 
 

look, these two little... people... have made an underhanded declaration of war. If you can afford to do the above, then your submarine will have just sunk their battleship. And until they're nice to you, at least keeping to the proprieties, Christmas presents will be cheap because, gosh, you just spent so much on that holiday, it was a splurge, but such fun!