My sister diagnosed as open angle glaucoma suspect in 1 eye has been given cosopt. Is this drop unusual as a first line treatment?

No. No. There are several excellent choices to initiate treatment. It's important to consider the patient's health, allergies, side effect concerns prior to selecting antiglaucoma medication. Also important is the desired iop lowering. With cosopt, you will get about 30% iop reduction, which is similar to prostaglandin analogues. I'm curious about "suspect" glaucoma diagnosis ; start of treatment...
Not unusual. But there are other medications that are prescribed more as first line agent. I also tend to watch suspects and only treat if they actually demonstrate glaucoma.