Star2015 I am really sorry you have had some very unhelpful and some downright rude comments. What a shame as mumsnet is for support.
I think Error404usernamenotfound has some good advice, ask your brother how you can be supportive and how he would like you to help your parents.
Mermaidhair1 great advice.
*Star2015, please can I ask how old your brother is? The fact he has not chosen to tell your parents before and that he didn't mention the relationship when it started probably tells you that he did know that your parents would not be accepting.
It can be a big deal for any family to find out any news they did not know, especially that to some degree your brother has had a life they did not know about a partner etc who they had not met. Although, of course, I am sure if the partner were a female it would be less of a surprise. They are thinking about this and are unhappy, whether others can understand this or not, this is the reality you and your brother have to deal with.
I feel very sorry for your brother, it must be very hard. Please do assure him of your wholehearted support and find some way to address any ambiguity you feel. Be aware he is also probably very happy to have met someone special. So join with him in this, meet his partner and treat him as you would any new special partner of your brother. His boyfriend is probably feeling very nervous too and hoping you will like him.
Be clear yourself, it must be hard - but you are not in the middle. At least you do not need to be. You can show a great deal of love and care for your brother and for your parents, listen to everyone, show love for everyone but make it clear you will not tolerate homophobic comments around you! Perhaps having to hold their comments to themselves around you will help your parents to be ready to be around you brother and his partner. Not tolerating any cruel comments is a first step, but really changing their hearts on this matter will take a while and ultimately only they can do it! So you do not need to be in the middle, you can be a friend to all but not tolerate cruelty.
Please ignore any rude comments. The fact other posters do not understand it or don't get it is really of very little help to you, of course! So they don't understand how a family may struggle with this information. That is fine but it doesn't help you to be able to help your parents to an accepting position. Just ignore the rudeness.