My daughter is gay? How obliged am I to homophobic in-laws?

I started dating my, (now) husband when my middle daughter was ten years old. When she was about 16 after struggling with her identity for a couple of years, she came out to us as gay. We supported her immediately and advocate openly for rights of all LGBT. However, I never discussed this or any other controversial subject during family functions with my boyfriend’s extremely conservative, Christian family. During the family’s political discussions (where everyone seemed to pretty much agree with each other), I always politely excused myself from the conversation and walked away. The issue of LGBT was never brought up during these discussions so I was not overly emotionally invested in their political opinions anyway. I observed my husband deal with his family and their extreme views in the same manner. I did however notice that around the time my daughter came out that my sister-in-law (to-be) became somewhat cold and distant. I wasn’t sure if I was being overly sensitive, but little details like receiving Duck Dynasty camping chairs as a Christmas gift (right around the time there was a backlash for their homophobic views) and not receiving invitations to her house anymore I noticed but may have gone over my head. Then our Save the Date e-mails went out. Within days of sending out our Save the Date wedding announcement a vicious comment was written on my face book page (where I had posted a news article about Texas lawmakers buying a cake to celebrate traditional marriage) from a joint FB account of my boyfriend’s sister and her husband about their feelings of “homosexuals” being an abomination, etc. I immediately began crying and called my husband. After talking to him, I felt reassured it could not have been his sister who wrote the comment and that as soon as she saw what her husband had written she would be horrified and call to say how sorry she was. Not only did that not happen, but within the next 24 hours she chimed in and together they quoted related bible verses, changed their profile picture to “speak truth” and just generally made it clear they were both in agreement with the comments that were made. My husband responded by making a FB statement that he would not tolerate anyone disrespecting his family or making homophobic comments. Although I had made a similar statement, my words had been ignored, but at my husband’s comment she immediately deleted her facebook page and called him on the phone. The conversation was not productive and she said she was sorry for the way we felt, but that was what she believed. After the conversation, I wasn’t sure how to handle her invitation. On one hand we wanted to go ahead and invite her, but my husband kept pushing the conversation about what we should do away. Days before our wedding she sends him a text saying that because she loves him and did not want to cause problems with me she would not be attending our wedding. I cried so hard that I caught a terrible cold the day before our wedding. My husband called her again and after talking to her for over an hour came to me and said she was crazy and they could not reach a resolution. This time his sister expressed that she had a responsibility to her son not to expose him to “homosexual” behavior and that if we were waiting on an apology one was not coming. She told him they had already made the decision not to attend our wedding and my husband agreed that would be best especially considering both my daughter and my daughter’s girlfriend would be there and would not be asked to pretend they weren’t together. I found out later that right before the wedding she texted other family members saying that although her brother wanted her there, he had to do what I wanted him to do, so out of respect for him she was not going to attend. This is not what he and I discussed at all. We were both on the same page of basically complete confusion about what we should do, although in agreement that we supported my daughter. I spent many days from invites all the way to my wedding crying because I knew I would never be able to pretend all was okay at future family functions. Although his sister still does not invite us to functions, his parents occasionally do. When I see her, she smiles and speaks sweetly and I try to do the same. However the strength and energy it takes to do so is so overwhelming I cannot continue to muster the will to do so. I resent that I’m spending my favorite holiday with people I have to pretend with and I am left with a migraine and energy drain that can last for days. I would never allow even my own mother to do and say the things to me that she did, so I’m having a hard time justifying forcing myself to be in her presence. In addition, it is nigh impossible to fake feelings, although it is apparent to me that in my husband’s family that not only would confrontation be unproductive, it would also not be appropriate or welcome. Although his whole family is aware of the situation, everyone pretends the whole thing never happened. Am I being fair in expecting the right to withdraw from family functions where I have to interact with her? And if not, how do I handle my body’s toxic reaction to being forced to repress my feelings during and after these interactions?