[stuck on sinnoh quest]

You should be able to rematch Pluton by going to Route 103. 

[No brother of yorkie to be found]

Can you send a screenshot of this location?

[No brother of yorkie to be found]

You should check next to the Power plant in Route 10, Yorkie's brother might be there .

[Not being able to walk when entering a new level]

Can you try options->Graphics and then select "Exclusive Fullscreen"?