What is the ruling on a young man proposing to the sister of the girl he loved and promised to marry after a long wait, with the sister being aware of this, and is the girl's action of boycotting her family due to her anger justified?

A man's proposal to a woman is legitimate and valid, but his mere promise to marry is not binding on either of them, and each has the right to withdraw from it. The woman should be content with Allah's decree and supplicate for success and for a righteous husband to be made easy for her. It is not permissible for her to be harsh towards her parents or to sever ties with her siblings.