Hello, I am Rafael. Thanks for asking your question - I'm here to support you. (Information posted here is not private or confidential but public).
This is a very challenging and uncomfortable situation for anybody in your shoes. I am sorry to know you are facing it.
There is not much you could do about what already happened. As you said they are her family, and the way her father left just empowered the attachment and expectations she has had for getting closer to them. On the other hand, it does not mean you have to repress your feelings and passively behold offensive or inadequate behaviors she may have. Setting healthy and clear boundaries is always essential for healthy and fulfilling relationships to develop and to take good care of their integrity.
it's good to know your fiance has been open and honest towards you about it, that is very good for sure, and based on the same honesty, you need to continue deepening your communication and mutual understanding, making it clear what appears to be unhealthy, undermining your mutual trust, respect and the fulfillment of your relationship. She knows how you feel, and needs to be consistent avoiding situations where she would get not to share exclusively as siblings, but any form of romantic or erotic behavior. Then you do not truly trust your fiance, right?
She was honest acknowledging how she felt the first time, and when she saw her picture, but has been honest about it. On the other hand your expectation shows you do nto trust her and that's why you do not want to spend anytime alone.
Have you had any previous episode where your trust was undermined or you suspected she was not being truly respectful and loyal?
If you see in her present behavior something that shows you she is betraying your trust, then such behaviors must be confronted and changed. If in the past she did present such behaviors, and that justifies your fears in the present scenario, then it would be wise to talk about it, since nothing guaranties it could not happen again. But if during the whole period you have been together, she has shown consistent respect, loyalty, and support to your commitment, then this would be a personal issues you need to work on yourself, rather than expecting her to change any behavior. Does it make sense?
I am sorry too know there were concrete episodes were your trust was broken, that explains why you feel the way you do, then I totally support your concern about present situation, and she should be able to acknowledge your feelings, be empathetic and work on developing an acceptable and healthy relationship with her siblings while respecting and taking good care of your relationship. You said you have made good improvements since then, then such good work must be consistently maintained and not neglected specially now that this obvious challenge has appeared.
If you try this and still find yourself unable to make things work, then it would be wise to consider individual counseling to work on it, and then couples counseling to work with her on the present situation, the behaviors that are undermining your trust and healing from past negative experiences and how to develop a healthy balance between taking good care of your relationship, while developing an acceptable and healthy relationship with her siblings.
I thing this extra effort is absolutely worthy since it is about two core dimensions in her life, your relationship and her family. It is not easy at all but tough, but I believe it could become a unique chance for each of you to make even bigger improvements in your communication, intimacy and trust. There is no need for this situation to create dilemmas as long as each of you keep clear understanding of and respect basic boundaries. it is OK to share with her brother, but not to get too close physically, like the situation you mentioned before. This is challenging and because of that it could potentially promote your relationship if you both know how to address it, with mutual caring, respect, caution and honesty.
Work on it, without repressing your feelings, but leave away any blaming, projection or manipulation. This way you would feel you are being truthful, respectful and supportive, while effectively coping with the unavoidable challenges this situation presents. Then time would show you how well it works, and which are the necessary adjustments or changes you need to do to make things work better. it would not help to focus too much on what happened before nor to fear the future, much more assertive to focus all your energy on making your present reality the best you can.
Every healthy relationship must be reciprocal, both partners sharing the same level of responsibility. No need to wrongly sacrifice anybody, if each hold full accountability for your choices and actions, keep a full honesty policy, and work on defining what is acceptable and healthy for you to do about this situation, then things should work just fine. If they don't, it would mean you need to get professional counseling support and time would show how really good match you do, since it is through facing challenges and problems that we find out how good match we create in relationships.
Nobody can, nor should try to control anybody but herself, and whenever it is about relationships, each person should spontaneously choose to work on self and on offering honesty, respect, affection, passion, understanding and support; they cannot be pushed, and in that way, healthy and fulfilling experiences would build a real and mature partnership.
I hope you could reflect on this , dialogue with your fiance and work together on it with all the support you could get to promote the best lessons and growth from it. You're very welcome. Thank you for your trust.