When I was 36 weeks pregnant with DS, I had to attend the wedding of an old school friend (and my DD's godmother) a good 5 or 6 hours drive away from home. DD was one of the bridesmaids, I was MoH, and I'd had a high-risk pregnancy for the first trimester. We spent the day after the wedding traipsing around local "sights", and whilst it was lovely to spend some time with DD prior to the birth, to remind her of how very loved she was (is), all I really remember from that weekend was being in excrutiating pain and bored from watching every other person present (DD was the only child invited) get drunk at the reception and wondering why on earth I'd let myself be talked into attending as a guest, never mind agreeing to be MoH... and how quickly I could convince (now ex-)DP to take us home again because I had cupboards to clean, and my own very comfy bed to sleep in, and washing to stare at as it dried... DS was born 17 days later, and whilst he still likes looking at the wedding photographs, knowing that he was the reason I stalked down the aisle looking like Morticia Adams (by that stage, the only "formal" clothes which fitted me, were my black maternity trousers and my neighbour's floaty top, not the beautiful dress I'd had to buy... and still it was "great" that I was pregnant and I "absolutely (had) to attend still", I'm still more than a little bitter about it. The only saving grace from that day is that my DD looked adorable and thoroughly enjoyed herself.
The 'happy couple', incidentally, were divorced within 3 years (it later transpired that they only got married because they fancied a big party

OP, my advice is... don't do it!!! You'll be uncomfortable at best, in agony at worst. You'll be bored, and tired, and there will be random people you barely know who'll genuinely seem to mean well, but will irritate you to tears. If you do go into labour, the bride may never forgive you for taking attention away from her, and you'll be 5 hours away from home and all its necessary comforts when you need them most. Plus, if your MIL is being somewhat precious herself now about what you need at a time in your life when you are at your most vulnerable, there's every chance she'll get a lot worse when your baby does decide to be born!
Good luck. I sense you're going to need a lot of it.