What is the meaning of seeing the fiancé's sister wearing a green jubbah, and she tells the dreamer that her brother, the fiancé, only gives what is beautiful, and this is after performing Istikhara prayer? And does this dream indicate the correctness of the Istikhara prayer?

It is not a condition for a righteous vision after performing Istikhara to necessarily be a dream or a feeling of comfort in one's heart. Even if neither of these occurs, and a Muslim performs Istikhara, takes the necessary steps, consults experts, and then proceeds, it is hoped that this will be good for him. It is a mistake for someone to wait for a dream after performing legitimate Istikhara. Even if a dream's interpretation indicated good, righteous dreams are merely glad tidings that cannot be relied upon. Rather, one should investigate and inquire about the suitor, and verify his religiosity and character. If these matters are confirmed, then the righteous dream serves as glad tidings and optimism for good.