This is a very sad situation, especially considering your 25+ years of marriage and I presume therefore, a long relashionship with this woman, your step daughter. Perhaps the most hurtful thing for you is to have become the person who is seemingly so despicable, has behaved so dreadfully (though your SD will not state what, exactly it is you're supposed to have done to have caused her reaction to you) that she refuses contact with her dad and cannot bring herself to even acknowledge/thank him for his birthday gift to her.
Now, I'm supposing you have not committed some unforgivable, heinous crime toward her and on the basis of that, I would suggest you do absolutely nothing. If you have done nothing to prompt this treatment of you and your DH then writing to her, paying for her to visit, wringing your hands over what you can do to get through to her, all of this is clearly falling on deaf ears. She is a grown woman with children of her own. If anything were to happen to her dad (and he's not a spring chicken, as it were) then that is NOT for you to deal with. It will be her sadness and only she knows if it will be worth it, this ostracising of her you both. She clearly thinks she has good reason for behaving as she does. You have been very ill and surely you're at an age where frankly, you ought not to be having this. Between you, you brought us 5 kids. They have all been through uni and done well; they're married with families of their own. You get on well with the others and you should be proud of what you've achieved, the two of you.
Let you husband continue his relationship with her. If she won't acknowledge you, unfair as it is, then let her go. There may be a time she wants you in her life again and then, it will be up to you.
You have done all you can then. The only other thing I can think of is writing to her to ask her to consider how she would feel if her father became ill and she had not put things right between them. She sounds quite unhinged tbh. Have her siblings been able to intercede with her?
I have done this cansu.I suggested he went to Canada without me but with her brother.She didn't reply.I set up dh with a separate email account as ours has been joint. He writes lovely handwritten letters to her at Christmas and on her birthday. He sent her a gorgeous Scandinavian silver necklace for her 40th.He has never had a reply or an acknowledgement.I thik her problem may well be with me.I suggested we pay for them to come over and I would go away for a week.It is a dire situation as she wont tell anyone what we are supposed to have done. Her parents break up was nothing to do with me.Dh's exw went off with a younger man and left the three kids with him.dsd was very supportive of our marriage.
On the basis of what you have written she is being very unfair and unkind BUT I would look at the bigger picture. You dh wants to have contact with his daughter. If her problem is with you, I would encourage him to contact her independently of you and teach out to offer his support. This isn't fair or right for you, but if your main concern is your dh then this is the right thing to do.
Dh and I have been married for nearly 25 years.When we married I was a widow with dc aged 13 and 17 and he was a single parent ft dad with dc aged 15 17 and 18.His first wife had left him for a much younger man and moved abroad. Over the years everyone got along really well.The boys-his two and my ds became very close friends.My dd was the youngest and had her own interests/friends but liked her step sibs and enjoyed their company.My dsd was 17 when we married and seemed very enthusiastic about the marriage although she chose to live with her grandmother so she could finish 6th form at her school. All five dc went to university.Dsd became distant but worked hard and achieved 3 degrees including a Phd.She married[all 5 married]and moved to Canada where we visited her 7 years ago when her ds was born. All the others had dc too. I was dx with breast cancer in 2006. While I was having chemotherapy and was very poorly dsd and her family visited[at our expense and her insistence].She was unbelievably horrible to me and said some wicked things.After 24 hours she left [obviously pre-arranged as a friend picked them up].Since then she has not communicated or responded to communications from us though she is in sporadic touch with her brothers.If they ask why she has cut us off she will not answer and cuts them off for months at a time.Her ds has been dx with ASD and we would love to be supportive.my dh is very very distressed by the situation.he will soon be 77 and is desperate to hear from her. I just dont know what to do. Any suggestions please.