I agree with Bridget D. I don't think it is particularly odd that Mary thinks this way about the implications if Tom were to die - indeed it would be more surprising if she did not.

I do find it odd, to say the least, that Mary would express her sentiments so explicitly to Fanny, and in such a tone. The line I find especially jarring is:
Fanny, Fanny, I see you smile and look cunning, but, upon my honour, I never bribed a physician in my life.

Mary is generally pretty shrewd in her reading of other people. She has spent quite a lot of time with Fanny and is well aware that Fanny is not like Mary's London friends. Of course she does not understand many aspects of Fanny's character, but how she could imagine that Fanny might "smile and look cunning" in these circumstances is beyond me. I find it very hard to believe that Mary could have misread Fanny quite so badly.