Hello nappymaestro - sorry to hear that your DMIL has had breast cancer, I hope she's making a good recovery.
I think after ten days, it might be a little early to know what her side effects will be - just because some things you get with a new drug are very short term until you get used to the drug. But longer term, the side effects that I have had (I've been on tamox. for nearly 3 years) were firstly menopausal symptoms, hot flushes, night sweats that sort of thing. I don't know how old your MIL is, - if she has already been through the menopause, then I presume she'll avoid those S/E's, (but I not certain - I'm not an expert!).
The other side effect I've had is aching limbs and joints, that seems to have got a bit worse the longer I've been on the drug.
I've also at certain times had swollen ankles as a side effect.
I'm sure other ladies on here will be able to tell you how they have been affected.
Tamoxifen is a hormonal drug - so if your MIL is on it, it means her BC is affected by oestrogen. It's a very good drug, but if she finds it doesn't suit (and I think some people can get things like bad headaches, dizziness and other things) there are several other equally effective drugs that she will be offered instead.
Of course I don't know anything about your MIL's DX, but by your description of her treatment, it sounds as if she is doing well, and that her cancer was caught in good time. (I base this on the fact that she didn't have chemo, - I did because my cancer had spread to my lymph nodes).
It is often difficult for people to discuss their experience of breast cancer, especially if they are by nature a reserved person. You and your DH sound like very caring people, and I'm sure she feels very supported by you.
She will have a breast care nurse, and that is someone who can offer her a lot of support and information if she feels she needs it. Also it might be helpful for her, and for you and your DH to have a look at the Breast Cancer Care website - they have a lot of information and leaflets you can send for about all aspects of the disease, aimed at patients and their families. 
Gracie - lots of luck for tomorrow, I hope all goes well for you, I'll be thinking of you. xx
Well done to DS Smee, - sounds as if he's doing really well
I am drinking lots (including some rather disgusting dandelion tea made for me by a friend who claimed it was good for 'wee problems')
Ned - petrol stations gone mad round here too, most are closed at the moment because fuel sold out, - bonkers!
night night all x