ayin jade

So much to say about this. Why do they disapprove of the marriage? From the sounds of things here, this marriage is doomed from the beginning. Altho the bible says a husband should leave his parents and cleave to his wife (Genesis 2:24), it sounds like his parents will always come before his wife. 
Which makes me wonder, why is he getting married at 35? Was he married before? If so, what happened then? If not then why so long a wait? Have his parents been a source of conflict with other women in his life? 
If you are both seriously thinking of marriage then you should get pastoral counseling first. From the sound of things though, it does not sound like he is serious about getting married. 
Sorry I know this isnt what you wanted to hear.

I would recommend praying for your brother (and your sister too) that they get saved. Getting saved is the first step to coming out of being a bisexual. He is lost in sin, like an addict, or an adulterer is lost in sin. He needs Jesus most of all. Get others to pray for their salvation.
Suggest that your dad speak to his pastor or a mentor at his church. I think it is best that he speak to someone if only for comfort and getting past the feelings of guilt he is having over this. Pray for your family, that they can heal from the pain over your brother being a bisexual.
Remember Jesus is the rock in your family. You dont have to be. Reach out to others in your church. Its why we are a body in Christ. Not alone. 

It was asked what the difference was. The difference is simple: one is of man, the other is of God. 

The Lord does not choose us on our merits. He chose moses despite what moses did. Just like He saves us based on His grace, not on our deeds.

I use capital H. Some versions of the bible use it. Others do not. I personally like to use it but I dont get huffy to those who do not. I do find those who call my Lord with a small g a bit offensive. Altho I usually wont mention it to them.