The way you've done it doesn't communicate 'in a large river' either. You'll need to pull back a little more to show the river's edge. I perceived it as 'close to two other islands' the way you sketched it.

Sometimes the right answer is "do more than one piece of art". You now have 4 different scaled items you want to include in this. It is impossible to get them all visible at one degree of zooming. To show that the island is in a river you must show both sides of the river and enough river to get the picture. In that case, I would put the insets not in the negative space, but in a large frame around the main map.