beccajoh I'm sorry about your latest news, that really sucks (understatement). But as everyone has said, it's a case of managing it. It's all very well for us random folk on the internet to say it, of course, but I'd take it very seriously from your medical team. They always err on the side of caution and do not make those comments lightly. I cannot imagine what you are going through, please vent here all you need, and cry/scream/rage, whatever you feel like. I wish there was something more useful I could say 
briteside I'm sorry you've had to join us, but welcome :) you will find lots of support here and between them, those who have had breast cancer have probably had almost every possible treatment combination, so there's bound to be someone who can answer almost any question! There's no point comparing treatment with others, even if their case sounds similar, as every cancer is slightly different and every team has different protocols/preferences etc. Your team will give you the treatment that they believe is best for you.
Lots of people find this early phase, after diagnosis but before a definite plan is in place, the most difficult. Treatment is a bit like a runaway train, once you're on you just have to go with it. That makes it easier in many ways as you will know roughly what is coming next, and there are few decisions to be made. You will get through it, but it won't always be much fun.
Is your mother-in-law someone who likes/is used to being the centre of attention? I had someone like that when I was diagnosed. When I told her I had cancer she said "oh yes, I know, I looked up your symptoms on Google" and "you're too young to be ill, you'll be fine", then back to her dislocated toe 
and how much pain she was in. Throughout my treatment she minimised things, unless it was how traumatic she was finding my treatment. Bizarre, but she is not in my life anymore! Bit more tricky with your mother-in-law who is presumably rather more permanent 
Otherwise perhaps she finds it too difficult to imagine you going through treatment so is in denial. She might get over it in the next few days/weeks - maybe your husband could talk to her? Ideally she would be supporting you both, or at least not adding to your stress!
I don't have any children yet but others will hopefully have advice about explaining to little ones - I think there's a book about "Mummy's lump" which might be helpful.
nelson I hope you get your results soon, it must be worrying. Presumably they are having to investigate the tissue sample very thoroughly, which must be quite time-consuming. In my experience, doctors can have a rather unrealistic view of how things work so if it was your consultant who booked the appointment it may be that she just didn't allow enough time for the standard lab procedures, not that the time has been delayed/extended if that makes any sense. Certainly my consultants have an air of clicking their fingers and expecting things to magically appear 
it must be hard not to expect the worst, have they given you an idea of what the realistic worst case scenario would be?I don't know if that would be helpful to you at all, but it might help allay some of your fears.
Waving to everyone 