What is the name of your state (only U.S. law)? Indiana
Can a boy almost 18 be charged with rape for having sex with a 15 yr. old girl and can her parents sue him/his parents for civil damages separately from any criminal prosecution?
The almost-18 year old can be prosecuted for RAPE if he compelled the girl to have sex with him through some manner of force or fear.
As it is, it sounds like this may well be at least considered as a matter of consensual sex and NOT "rape." As previously mentioned, if this were RAPE and the victim reported it, the suspect might well be in cuffs now. If, however, she says "rape" and he says, "consensual" then it may be impossible to prove a forcible rape situation absent a rape kit and examination that showed that the elements of forced intercourse existed. Lacking any evidence of course, or witnesses, or other evidence, then this would appear NOT to be rape.
Suing someone for civil damages would be difficult. You'd have to prove by a preponderance of the evidence that the suspect committed a forcible rape. This is not likely unless there is evidence of the crime to begin with.
As for suing the parents of the teen, you'd have to prove they're culpability in the assault. Merely being in the home may not be sufficient. And, again, you'd have to prove that a forced sexual assault occurred.
Before you start thinking of a civil suit, CALL THE POLICE!! The longer you wait, the harder the case will be to prosecute. Already, bedding and clothing that might have been evidence of a crime has likely been washed or even disposed of rendering them useless. Unless he admits to the rape or acts consistent with rape, this is not likely to go anywhere.
Instead of thinking of a civil suit, contact counselors for your daughter. She will need help - not money.