When Arima's extended family visits the house and verbally harasses him (as usual) his foster father stands up for him and declares that no one should speak ill of his beloved son. In the second half of the episode Yukino and Arima are able to spend some time alone and bring their relationship to a higher level. This level of closeness causes Arima to have a nightmare about his past.
January 29, 1999
And yes, they basically have intercourse :]