Here are three articles found in
the Index that your sister and her boyfriend should read and understand:
I'm Ready for SEX! (or am I?) I really want to do it.
What Can I/We Do To Prepare For First Time Intercourse?
Making Out--Knowing How Far To Go
To inform or not to inform your parents about this is a difficult situation. Here are some things to consider:
a) that you know she is using the pill or some other highly reliable
form of birth control
b) that you know for certain the b/f is using a condom
c) what are the house rules or expectations
If you are older, I would suggest asking her about how responsible she is with her own protection and in requiring the b/f to be equally responsible. Both of them need to look out for "Number One" (themselves) and then use a third form of protection as back up to the other two--a spermicide. Teenagers are more likely to become pregnant than any other age group and it behooves the couple to make use of all three. If not the pill or patch, then at least the other two.
Ask both to read these articles more than once and to discuss between them what they've learned.
As for informing your parents, only you can be the judge of this and whether or not it is the thing to do.