Ticks!

Type 1 diabetes doesn't always have a family history even though it plays a part. So if your father or brother has it etc you are at a higher risk but it isn't seen as hereditary. Your genes along with environmental factors play a part. So your DIL would have been carrying the gene and then something triggered the auto immune response -just as PP has said, she believes the Lymes disease triggered her rheumatoid arthritis.
Both my sons have T1, we don't know anyone else on either sides of the family that also has it. Mine were both diagnosed in the same week ( twins) I believe it was triggered by a very bad sickness bug they both had after they swam in the river. It is a well known river swimming beauty spot but there were several sickness cases that summer. They were both very poorly. Then 5 months later they both developed T1. My husband has auto immune issues which I believe runs in families which is often the "link". He is coeliac and has other auto immune problems - but coeliacs seem very much linked with T1. Often in families one child will be T1 and the other will be coeliac. You may well find that your DIL has family with other auto immune issues.

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I would read about Frontline before using it on any animal. We used it on our two bichons, one was fine but the younger one began having seizures! Having researched it, there appears to be a link with Frontline and seizures, in fact I’m sure there was going to be a class action in America against the manufacturer. We stopped using it and just took our chances. Luckily we had no fleas, ticks etc during their lifetime (16 and 14 years).

Collies are a bit strange with tablets/injections too. The vet is a bit anal with giving them worm treatments for that reason.....but I've had another 4 off him today - all on the underside of his mouth. He hates me at the moment. My tic tool came today. And I rang the vets earlier as it all seems to be prescriptions only. So going for the tablets that she has suggested.