Sandy is Soda's girlfriend. She isn't involved in any of the present action of The Outsiders, and she remains behind the scenes the whole time. Pony has a pretty good impression of her, unlike his impressions of most of the Greaser girls he knows. When Pony learns that Sandy's pregnant – not by Soda – and that Sandy's parents refused to let her marry Soda, he realizes Soda has problems too. Since we only end up hearing this information from Darrel, some readers wonder if Sandy's really pregnant, who the father is, and why she was sent out of town. Here's most of their conversation:
[Darrel:] "When Sandy went to Florida… it wasn't Soda, Ponyboy. He told me he loved her, but I guess she didn't love him the way he thought she did, because it wasn't him." (12.39)
[Ponyboy:] "You don't have to draw me a picture." (12.40)
[Darrel:] "He wanted to marry her anyway, but she just left." (12.41)
So, why do you think Hinton was so vague about this point? She obviously doesn't shy away from talking about controversial issues in the rest of the book. There could be many reasons, but we think it's probably to keep Pony and Darry in character. Neither of them is comfortable talking about the issue, and they sure aren't going to come out and use words like "pregnancy." As Hinton shares, this has much to do with the setting, the 1960s. In trying to answer readers' questions about Sandy, Hinton provides a little background information:
[…] [Sandy's] parents sent her to live with an out of state relative due to her pregnancy, which was obviously 'out-of-wedlock' and was not socially acceptable at the time. Social attitudes have much changed over the last 40 years; thus making it easier on unwed mothers to not only have (publically) but keep their babies. It has not always been so, as is illustrated in this book (although between the lines). Up until the mid 70's or so, if an young woman came up pregnant and unmarried, they typically were sent 'away' to have the baby. […] Many times they were sent to Unwed Mother's Homes until they delivered; leaving their baby behind and up for adoption. (source)
So, in addition to helping us gain insight into Soda, Sandy's character represents a serious issue for women at the time.