Through the back window of their house, Sal sees Mr and Mrs Finney lying down on the garage roof. They hold and kiss one another and Sal says that "It made (her) feel peculiar." There are a few reasons why she felt this way.
The first reason is the one that Sal herself states explicitly. Mr. and Mrs .Finney remind Sal of her own parents, before "the stillborn baby, before the operation." After the operation (a hysterectomy, meaning that Sal's mother couldn't have more children), Sal's mother disappeared. Sal feels peculiar when she sees Mr and Mrs Finney being intimate together because it reminds her of a time when her parents were happy and intimate together, which in turn reminds her that they are not happy and/or together now.
On a slightly deeper level, the peculiar feeling might also be something like guilt. Since Sal's mother disappeared, Sal hasn't spent as much time with her father as she might have. She perhaps realizes, watching Mr and Mrs Finney together, how lonely her father must be and possibly feels a "peculiar" sense of guilt because she hasn't thought too much about that before.
Another reason for Sal's peculiar feeling here is that she sees Mr and Mrs Finney being intimate during a passage in which she is with her own romantic interest, Ben. Just before she sees Mr. and Mrs. Finney, she is speaking with Ben and she says that she "liked the way he looked right in your eyes when he talked to you." She also soon after says, as Ben reaches into a cupboard for a glass, that she "had that odd sensation that I wanted to touch his face," and that she "was afraid [that her] hand might just lift up and drift over to him if [she] was not careful."
Seeing Mr. and Mrs. Finney being intimate together makes Sal feel peculiar because it draws her attention to that "odd sensation" of romantic, slightly awkward teenage love that she feels for Ben.