Like Michelle M., I also wondered about your comments on the brother. What do you mean when you say he "puts pressure on her"? How exactly did the brother "let" her see some frightening thing on YouTube "accidentally"? Your use of the quotation marks around that last word indicate you know it was no accident. Do you think he bullies her to do what he says? Does he live in her mother's home with her all the rest of the time she's not with you? Do you have or know of other issues with this boy and his behaviors toward younger kids? I would get that sorted out and as Michelle says -- maybe be sure that visiting you is a break from him and he is not with you at the same time she is there.
Is the girl, herself, in any counseling? Going by what your own counselor said is OK, but I wonder if she has generalized anxiety that reaches beyond the issue of home visits. She sounds as if she might have problems with changes and with making transitions; she might also be jealous of the younger kids, or feel that when she's at your house she is the "odd one out" (at her age, no matter how hard you try to include her, she may feel left out because at 10, kids sometims just cannot re-set their feelings no matter what adults do to reassure them). She may even be at a stage where she is keenly aware that many of her friends have parents who are not divorced and siblings who are not younger half-siblings -- in other words, 10 is an age when kids really begin to realize how their family is different from other kids' families, and though many, many blended families exist now, she may be pining for some ideal she has in her head.
All that points to possibly getting her some counseling of her own to find out the roots of her anxiety.