I’m sorry this has happened to you and your children. It is very common to find parenting too hard when you have a child very young, or if you haven’t been parented yourself because you are in the care system. Without support, it is very hard.
If your son is with the same family as your daughter, and is regularly coming every two weeks, then I think that the grandparents are probably trying to encourage a relationship from their end, but that it isn’t working for your daughter.
It sounds like you have done a great job of getting set up with an adult life with a house and a job. That’s brilliant.
I think that it has probably been a very difficult thing for your daughter being removed from you and growing up with her grandparents. She probably loves you a lot but has complex feelings about you not having been able to keep her. It is good that she is doing therapy.
In your position I would try to consistently have some sort of contact with her every two weeks, on the same days you have her brother, so that she can see some consistency in what you’re doing. And like pp have said, if she doesn’t want to see you I would make that contact in a form she is happy with.
Examples of things which you could do are text her, write her a postcard, write her a letter, call her (these seem to be going well), if you buy a food treat for her brother get her one too, if you buy a little gift for her brother get her something too, give her a book you liked when you were 9, say hi when you drop off or pick up if this is allowed. You don’t have to do the same thing each time.
The other thing which I would do is, if you’re on the phone with her, I would take all of the pressure about seeing you away. I’d say something like “grandma says you don’t want to see me and I want to let you know that’s ok. I’ll always want to see you, but I’ll never force you to see me. And if you change your mind in future we can see each other however you want, wherever you’re comfortable with”. Sometimes removing the pressure let’s a child make a different decision.