What should I do after performing Istikhara for my marriage to a girl, when I saw nothing in my dream and felt neither comfort nor unease, given my hesitation due to the short stature of her father and brother, which might affect the height of my children?

Repeat the Istikharah until you feel a sense of ease, keeping in mind that the short stature of the girl's father and brother does not necessarily affect the height of the children born from her. What matters is your desire for the person of the righteous woman, and marrying her is not considered an assault on the progeny.