Despite all the proposals my transcription has prompted, I have incorporated most of them by consensus. That is the reason why three words are still displayed in brackets since such expressions require further analysis. Therefore, I would be glad of subsequent suggestions.
My dear Sister ye 14th of feb
1766
I intended you a longer letter tonigh
but behold M ss Carter at my elbow, looking as if
she had a great deal to say, which is certainly
singularly malicious, as she is of all Womankind
the most silent. I will certainly take care about
M ss Larghornas son. When I had written this in
came Lord Lyttelton (so/as) I was obliged to lay down
my pen, for she Mss Carter with it disposed to
talk to me, just when the company increased
to three persons, I knew tho she can (……) or
dwells she wd not venture to engage in a trio
(….) poor Lord Lyttelton must have
furnished more than his share of the
conversation whereas when we parted she
Late Modern English letters 
Dear beayjulio22,
My transcription is like yours except a few differences which are In the first sentence ‘tonigh’ instead of your (wrigh), and further in
Line 4: Womankin(d)
Line 7: Lord Lythelton
Line 8: for ‘she’ Mss Garter looked disposed to
Line 10: tho she can sing a
Line 11: duette she used not venture to engage in a trio
Line 12: as poor Lord Lythelton
Dear beayjulio22,
Although our transcriptions are very similar, I also agree with Maaike1952 about tonigh (l.1) and womankind (l. 4).
I think line 7 should be Lord Lyttelton (see http://www.jstor.org/discover/10.2307/2914523?uid=3738736&uid=2129&uid=2&uid=70&uid=4&sid=21101091064523) and after Lord Lyttelton I read the word ‘as’.
I agree with Maaike1952 about line 8.
Then in line 9 I read ‘just when the company increased to three persons …’
Again I agree with Maaike1952 about line 10 and then in line 11: ‘duette she wd not venture to engage in a trio …’
The rest I agree with.
Dear beayjulio22,
Although our transcriptions are very similar, there are a few differences.
Line 1 and 4 I agree with Maaike1952: tonigh and womankind.
Line 7 should, I think be Lord Lyttelton (see http://oasis.lib.harvard.edu/oasis/deliver/~hou01749).This refers to a certain George Lyttelton,
Then after Lord Lyttelton I read the word ‘so’.
Then again I agree with Maaike1952 about line 8 and then in line 9: ‘just when the company …’
In line 11 again the same as Maaike1952 and I agree with you that line 11: ‘she wd not ‘
regards