Already posted 

http://sherlock.boardhost.com/viewtopic.php?pid=23298#p23298

It's lovely, isn't it?

So perhaps Sherlock was speaking from personal experience when he laid into the fisherman wearing the awful Christmas sweater in Hounds??? 

And the expressions on their faces... 

I've taken that dialogue to mean that Mycroft always assumed he had the right to control and direct Sherlock ( as the older brother,   as the smarter brother,  more responsible, more reasonable, whatever- and then, much later, as the brains behind the British Government). And Sherlock always resented it and went against Mycroft's wishes quite a few times; thus the old friction. The fact that it is wrapped in  a word-play is the icing on the cake and one of the things that makes "Sherlock" spectacular.

The Doctor wrote:

.. watched a few excerpts of Scandal with an uninitiated colleague who is quite young - and ended up discussing dialogue. And I realised I never quite thought about Mycroft's "I'll be mother". So I checked:

It's normally heard as Shall I be mother? meaning 'Shall I pour the tea?' It's used because pouring the tea has traditionally been seen as a mother's role. I suspect it's now heard less than it once was for various social rather than linguistic reasons. It's not slang and it's not facetious, but because of the nature of tea-drinking it's likely to be heard in informal situations.

Thank you so much for clearing this up for me. I was never quite certain what he said or what it meant. I love the British terminology they throw in, lean something new every day.