Children don't accept difficult new circumstances, they just tolerate them. This "children are adaptable" nonsense is something adults tell themselves when they choose to upend their world. I mean, they don't die from most selfish adult choices, and they will adapt in the sense that anyone adapts to things that make their life more difficult - but it doesn't mean they will ever be happy about it.
What children will adapt means is children are powerless and forced to tolerate bad adult decisions. Bad adult decisions don't improve over time, but children learn to shut up about their unhappiness because they have no choice, or get angrier and angrier over time.
I suppose there might be the very odd rare case where kids actually are pleased to share their lives with strangers and half siblings, anything is possible.
Being forced to share resources and time with strangers is very unpleasant and upsetting to adults, let alone powerless children. I have never seen it work out well in the real world, and I have known four blended families who had the best of intentions.
Honestly, on this site it's apparently child abuse to raise your voice to a kid because they will be traumatised for life but by then when you turn around you're being told they will adapt to their whole world being upended forever because - well, because it suits some people to believe so.
The saddest tale I ever heard about kids and sharing their parents was from a man I used to work with, a really nice kind man called John. He and his wife had fostered two teenagers while his own kids were teenagers. The fostered teens were not violent or badly behaved, but of course they had needs as all children do, particularly ones who have been lost their parents.
John and Viv did this with the best of intentions, they truly thought it was the right thing to do. When his son and daughter were in their 20s his own children informed John and Viv that they had thoroughly detested every single second of being forced to share their love and their home and their resources with children they didn't want to know and weren't related to.
They resented it deeply, and possibly forever, I haven't seen John in a decade, so who knows how it all panned out, but they had been holding on to their anger and resentment for years.
John was so shocked by this news, as his kids had always tolerated the circumstances with good manners and decency and appeared to "adapt".
He was in his 50s when they told him and he came in to work one day upset and told a couple of us who got along well with him. He stated that he wished he had not fostered the other kids, because he permanently damaged his own relationship with his own children by forcing them to share love and resources without outsiders.
Now, no matter how much anyone reading this feels the need to launch into a lecture about the sainted angels of foster care and how society needs them - this is a true story and one I am telling here in the hope that the OP will realise that it's not just her kids' lives she is going to make more difficult and less happy.
I'm aware that fostering and blending is different but for John's kids the outcome was the same - actually better since they at least didn't have to put up with strange adults in the mix with power over them as blended kids do.
They won't forget that the OP chose to inflict another family on them. The OPs kids are utterly powerless and there is no plus side for them to this arrangement the OP is planning.
I remember years ago reading a parenting book that explained it quite well. Imagine your husband came home and told you that he'd found you a friend, a companion, a lovely woman to share your chores with and chat to, and she was going to be his second wife. Don't worry darling, you'll get used to sharing, you'll adapt, in the end it wil be the best thing, you'll see.
That's how most kids feel when they're told they have to share their home and lives and resources with usurpers. Many of them lie about it to placate their parents because they are afraid of the repercussions. Some act out. None are happier when forced to share with strangers they didn't choose, though yes many will tolerate it without too much fuss.
Blended families are about the parents not wanting to be alone - in the OPs case she has snagged a good un and wants to hang on to him.
Ok, that's her choice. But don't think it's going to be a fairy tale ending.
And I couldn't read another pointless "kids are adaptable" platitude without responding to that realistically.