From the script (emphasis mine):
She turns on the bedside lamp. It's the same girl George was spying on, and Marty recognizes her just as we do...
Marty: Oh my god. You're... You're my... You're my... my...
Lorraine: My name's Lorraine. Lorraine Baines.
Even without this information, it seemed pretty clear from the scene itself.
When Grandpa Baines hits Marty, he gets out of the car and shouts to his wife. This means that the accident happened directly outside the house. In fact, 1985 Lorraine McFly said that George McFly was hit by her father's car in front of their home. The girl was in an upstairs bedroom window. We know that Lorraine's bedroom is upstairs, because when Marty wakes up, she tells him he is in her room; then Marty falls out of the bed, and we hear Grandma Baines shout "Lorraine, are you up there?" The woman in the window was very attractive, and fairly young, but certainly not a child. Grandma Baines is not attractive, and there are no other women or girls of roughly the right age in the house. It must have been Lorraine.
I think we're supposed to find it funny that Marty's dad used to peep on Marty's mom. It is also a bit creepy, of course, but therein lies the humor. Marty's dad is a pervert.