My father was diagnosed at 26. I was diagnosed at 21. My youngest brother at 24. Another brother was diagnosed at 12. None of my 5 sisters have shown symptoms.
T1 is one manifestation of an autoimmune condition. To me it is the condition that is spread in your familiy not T1 itself. By looking closely at your family tree you will find all sorts of rare autoimmune problems: thyroid, skin problems and so forth. T1 is just so rare because the likelyhood that your body will attack the little mass of beta cells is not very high. The speciality of T1.5 is just the slower development. This slower progression may also be influenced by inherited properties of the immune system.