Definitely give up on the idea of expecting an apology.
She dislikes you for whatever reason (pipped her to the post in getting engaged maybe?) and won't tell you those reason(s). As a result you, understandably, don't like her.
You should be capable of being in each others' company on family occasions and not show animosity towards each other. (I know that is easy to say but will be awkward to do in reality).
I think the problem is really caused by your fiance's parents and brother in that they feel awkward because they know of the situation and then they behave differently.
You need to stop discussing any issues with them, other than to say that, although you don't get on, you will behave civilly and kindly towards future BIL's gf. You need to be the bigger person - 'kill her with kindness'. It is hard to be nasty to someone if they are genuinely being nice to you.
I'd suggest OH has one last chat to his brother saying you are going to be polite and friendly towards his gf and expect the same from her in order to move forward. Say that you wish to move forward and get on as well as you can. Also OH needs to find out where his DB wishes to sit at the wedding, if he is already aware that the plan is for him to sit at the top table. There is no way I would ever have the gf at the top table, feeling as you do about each other.
Perhaps pitch, as nicely as possible, the suggestion that, as he is not the Best Man, he sits at a table for guests with his gf so that she can enjoy his company. Give a deadline of a few days for him to come back to you about this.
It may be that gf does not want to come - which would actually be a bonus for you (don't say that to him). If she doesn't want to come then just express polite regret and say you'll contact the venue to reduce the numbers downwards. Hopefully that will ensure she does not change her mind.
Whatever you do, try not to act gleeful as presumably you want future BIL to be there for your OH.
Focus on all the positives in your life and don't let the negativity drag you down. Have a fabulous wedding day and a very happy life together.