Is ghusl obligatory for a girl who felt aroused when her brother stood behind her, then found a light yellow fluid after two hours, knowing that she did not feel pleasure?

The cure for obsessive thoughts is to ignore them and turn away from them. If the discharge of a fluid is doubtful, the original presumption is that nothing has been discharged. If you are certain of its discharge, ghusl (ritual bath) is not obligatory unless you are certain it is semen, which necessitates ghusl. When in doubt between semen and other fluids, a woman may choose and assign to it the ruling she wishes, and there is no blame upon her as long as she is not certain of the discharge of semen, which necessitates ghusl.