Don't expect too much blood or to see the waters break. It's quite rare for floods of water to come out and sanitary towels are usually kept close by to soak up the blood after the show.
You will probably be sworn at a bit and told to generally leave her alone, but just stay by her side.
Expect to have to go to the toilet with her (and expect some mess on the bed) as she most likely won't be able to make it by herself and mid-wives will be backwards and forwards a lot of the time. Also, she'll probably get really hot so expect to see a naked mother
Don't panic if there are complications. My son's heart beat started dropping and had to be pulled out. Forceps didn't work so they had to use a ventouse. The baby came out blue, cord around it's neck and not breathing. At this time there were about 8 people in the room (me, mother-in-law, girlfriend and the rest nurses). The fact is they are all professionals and know what they are doing. The most important thing you can do in that situation is to reassure your mom (and yourself) that everything is ok.
I would prepare some sandwiches and some bottles of pop (Loads of water for your mom too, as well as some cooling spray if you can). I was in hospital for around 15 hours, you may be in a lot longer...
If your she struggles towards the end, you may need to assist the mid-wives by pushing your mom's shoulder up off the bed when she is pushing.
I would recommend watching the baby coming out (if you're comfortable with it). I know it was the best moment of my life
[/YET ANOTHER SPUNKY LIFE LESSON]