How do I (40F) handle the adult child of my fiancé (55M)?

I just got engaged to my fiancé last week. He text his three (plus one DIL) kids about it and three responded well. They were all notified well in advance of his intentions with me. The only daughter (25) did not respond and ignored the text.

Yesterday I went to his house to help him as he just had a tooth extraction. His daughter came down mid-afternoon after sleeping in and we were on the couch watching tv and looked over and said good morning and started talking to her but she had her airpods in and ignored us. So fiance got up to say hi and let her know we were talking to her but she refused to talk to him and proceeded to eat at the kitchen island, ignoring us for 20 minutes while she did her thing in the kitchen (open concept with living room for imagery). He came over and said, "she's just a moody teenag-" caught himself saying teenager. And I said "teenager?" and he was like no she's a grown adult as if he were reminding himself. He said she's struggling with the changes (We are moving into a house next month about 45 minutes from his current home where he will allow his daughter to live as long as she wishes and is encouraging her to get a job and pay rent to cover the house tax/internet so she is struggling with the idea of working).

I did tell him that I don't want to go anywhere I'm disrespected and he understood. I feel like an asshole for telling him that trauma doesn't give you a permission structure to be an ass to everyone (I experienced a lifetime of trauma in my childhood so I say it with conviction) and expect everyone to accommodate her moods. He's working through things with his therapist, too so I definitely know this is a tough thing for him, too. I know he's ready to move on with the second half of his life, feeling rejuvenated with how much energy I bring.

His daughter has been living with him for a couple years now and going to the community college part-time for art classes. She doesn't pay any bills, rent, tuition, etc., and doesn't have a need or desire to have a part-time job since dad takes care of everything. He even had a house cleaner until last month so she really didn't have to participate in the well-being of the house. She does struggle with anxiety, and I do feel empathetic for the whole family since they lost their mom/wife 6 years ago. He did everything he could to give them therapy, normalcy, and a chance to succeed in life despite their devastating loss.

So I know it's a difficult situation to marry into. The other two boys live on their own, one with a daughter and wife, and both are doing very well in life and their careers. Those three have been very welcoming and supportive of their dad finding happiness and a new life with me. The eldest even text me saying welcome to the family, we know you love dad so much.

The "kids" have mentioned "walking on eggshells" with the daughter and find it hard to talk about changes around the house like wall paint color. It took her 6 months to just come downstairs to meet me (last year's easter was awkward with her refusing to come down and eat with everyone). I know this has to be hard for her but it feels wrong that she gives her father the silent treatment while living under his roof/care.

I will see him today and he will apologize for his daughter for ignoring us yesterday.

I figured I would post here and get some hard truths or advice bc I love my therapist but she doesn't do advice well, lol. I don't need to rush her healing process but I also don't want to fall in line and cower to her demands and moods like everyone else in the family.