I've had (and survived) cancer myself, and have been through close friends having cancer, and my cousin is currently terminally ill with it, so sadly lots of experience.
Firstly, yes, cancer (or any serious illness) will bring out a host of emotional vampires. They feed off health problems, disasters, and love having gory details. Give them an inch and they will insert themselves like bloodsuckers. Don't give them the inch - politely but firmly tell them to Fuck OFF. One of the nice things about cancer is that you quickly develop the ability to do this without worrying.
Secondly, there's a whole group of people who are genuinely and desperately concerned about your mum, and are wanting to help. Best advice - set up a blog with privacy settings and invite all relevant parties to the URL. Explain that ALL relevant health updates will be posted there. In this way, everyone gets a chance to know at the same time and you and the immediate family are saved from the emotionally draining and tiring job of phoning to give the same information several times over. In the blog, you can set out things like Mum has been today to see her consultant. She is likely to be having an MRI and a CT scan next week, are there any volunteers to drive her to these appointments? This way,s you can start sharing out the load and believe me it is a load, and people do genuinely want to help.
However, DON'T wait for people to magically know what needs doing. You need to tell them. So this can be things like mowing the lawn, getting in groceries, accompanying your mum to appointments, changing her library books, etc. Spell these out in as much detail as possible - you'll find that folks will be really glad to have something specific to do rather than the vague and annoying "is there anything I can do to help?"
Other tip - which sounds horrendously ungrateful - get them to co-ordinate sending flowers. I was inundated with flowers when I came out of hospital - there were so many it was overpowering - and we had to buy extra vases and they were everywhere. Far from being welcome, the sheer amount was awful. However, later on in my treatment, there were lots of times I'd have loved flowers ot cheer me up.
Make sure your dad is being looked after - possibly ask one of the male members of the family or one of his close friends to make sure he gets a regular night off or the chance todo something he enjoys. The main carer has a tough job.
Good luck to you all xxx