What is the way to stop thinking about a particular girl, and to stop awaiting praise for good deeds in front of her, given the suspicion regarding one's intentions when playing with her younger brother?

The way to stop thinking about a girl you do not intend to marry involves two things:

1. Emptying: Severing all ties that connect you to her or remind you of her, even if it means distancing yourself from your friend. Nothing is more valuable than safety in one's religion. The Prophet, peace and blessings be upon him, said: "I have not left behind me any trial more harmful to men than women." And whoever guards against doubtful matters has indeed cleared himself in the matter of his religion and his honor.

2. Adorning: Occupying the heart with what fills its void, from the things Allah loves, such as acts of obedience, worship, and remembrance. A heart preoccupied with what Allah loves will not be preoccupied with its opposite.