[oops, wrote this ages ago and never got around to posting it]
Interesting, I've never thought about that. The following seem to work:
I am/we are
I am not/ we are not
am I not?/are we not?
aren't I?/aren't we?
But not:
are not I?/are not we?
am not I?
OED says 'aren't I' is a relative neologism, and cites:
1907 LADY GROVE Social Fetich 38 If ‘ain't I?’ is objected to, surely ‘aren't I?’ is very much worse.
1934 DYLAN THOMAS Let. 2 May (1966) 117 The first thing I would do..would be to peep, with a nasty aren't-I-a-lad expression, into the pages.
1946 K. TENNANT Lost Haven (1947) ii. 44 ‘Oh, Christ!’ Kelly moaned. ‘Aren't I telling you I aren't deserting you?’
Apparently 'am not I' was originally used, but it feels wrong today. Oh well...