I don't know of any research into this subject, but three of my six
children are diabetic.  I am fairly sure that there is no direct link to
this having been caused by cow's milk as they were all breastfed and had no
cow's milk at all untill relatively older.

Type 1 (or insulin dependent) diabetes in children is caused by an auto
immunological deficiency; or so the thinking goes these days.  Personally,
I would think it highly unlikely that milk drunk by a diabetic mother
during pregnancy would cause diabetes in her offspring.  It may have other
effects, but for diabetes to occur there would have to be a tendancy to it
which is probably genetic anyway and the link could be quite tenuous.

I would be interested to hear whether anyone knows of research concerning
diabetic mothers and breastfeeding.