ANSWER:

Dreams have no impact on the Istikhara.
Istekhara should be done by the one who’s getting married. 

Kindly refer to the following link(s):

Yesterday was the first day of my istakhara. I was doing it for my brother’s marriage proposal. I saw in dream that my dearest cousin(who is more than a sister to me) is dead along with her daughter. What does this dream mean? Should I continue the istakhara till 7days or should i stop doing it? What should i do with the proposal?