I am sorry to hear about your Mothers diagnosis. This will undoubtably be a difficult time for you and your extended family. Try to remember that she is your Mum but she is also their sister/ daughter/ wife.
Devils advocate point of view here.
On one hand your mother has an absolute right to confidentiality however it is entirely possible that her wishes in this regard are depriving your Father of the support that he needs at this time.
This may have been why he chose to tell people.
My mother was the same. We respected her wishes for many years but in the final 6 months my Dad, siblings and I needed as much support as we could get so we told people and started asking for help - groceries/ meals/ finding a cleaner etc. . She was ill for almost 20 years, my Dad cared for her at home for 10, she was virtually housebound for 3 years and for some (close) friends this was the first they "officially" knew.
You will never understand families. I have always had a good relationship with my cousins (who now live away) yet since my Mother died in January neither of them have been in touch to find out how we are. In the run up to my Mother's death they were useless. The final events of 2012 kind of sum this up - my Mother took ill (again) on New years eve, my Dad had an "this might be it" chat with the medical staff when he was in visiting and once again discussed her DNAR/ Not for ITU status. He then passed this information on to close family by a combination of phonecalls/ e.mails.
My last conversation of 2012 (coincidentally I was ill in bed and had been for 5 days with a chest infection) was with my brother deciding which of us would drive to the hospital if we got phoned in overnight as Ma was dying.
I was therefore decidedly unhappy to get an e.mail from my cousin the next morning (she knew I was ill and had received the same e.mail we had from my Dad the previous night) asking if I'd had a fabulous New Year, she had had a party - it had been great and containing 10million photographs of her children.
Insensitive doesn't even really cover it. So basically do what works for you. Put your own support mechanism in place. Remember that your Dad is also grieving and will need his own support.
If the family aren't being helpful tell them or exclude them.