How do I pick an instance by variable(string) in expression?

No, that's not possible. The only way to refer to a specific instance inside an expression is by IID. For example, Sprite2(9).x where 9 is the instance IID.

You can also make a function which will pick Sprite2 instance and return its x or y. In this case the expressions may look like this:

Sprite1 set position to X=Functions.getSprite2X("right"), Y=Functions.getSprite2Y("right")

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Instead of System Pick condition you can use Sprite Compare Variable condition - it works the same way, but will look better in the event sheet.