1. Is the lack of desire to marry due to fear? 2. What is the appropriate supplication for someone who feels jealous of his younger brother and wishes him happiness?

For someone who has reached the age of marriage, it is natural for them to desire it. If this desire is absent, it is an abnormal condition with underlying causes. These causes may be organic (such as weak sexual ability), necessitating consultation with doctors, or psychological (such as fear and apprehension), requiring advice from mental health specialists.

As for the mentioned jealousy, it is either ghibṭah (envy in a good sense), which is wishing for something similar to what another possesses without desiring the removal of their blessing. This is the state of a believer. Or it could be ḥasad (malicious envy), which is dissatisfaction with Allah's decree and wishing for the removal of a blessing from another. It seems that this case is closer to ghibṭah, as the questioner wishes happiness for his brother. There is no specific supplication for this, and one can refer to Fatwa (definition) No. 32558 for overcoming ḥasad.