I remember reading from somewhere that Finnish and Estonian were pretty much the same language until about 14th century AD or so. Nowadays these languages are quite different, about as distant as English is from Frisian. This issue was debated here about a year ago.  Estonian is supposed to be closer to proto-Balto-Finnic than Finnish, or so one linguist have told me. I am no expert in this... Yes ancestors of modern day Finns and Estonians spoke same language, but since Finland became colonnized by Swedes, Estonia by Danes and Germans and Karelians by Russians, these languages started to evolve separately.    Still Finnish is more archaic language than estonian what comes to pronouncing and grammar. Estonian is more simplified and worn out. Also Finnish has tradition of inventing own words for new things when estonian has more often just adopted foreign loan.    I wouldn't compare difference between Finnish and Estonian with Frisian and English since we can understand each other if we want. More likely difference between Dutch and German is more equivalent. Northern Finn and Southern Estonian wouldn't understand each other but Southern Finn and Northern Estonian do it little better.