Talk with your hubby, get on the same page, and then talk with each girl separately and then with both girls together.
Talk with your 14-year old and see if she needs to talk about anything that upset her--do not grill her on what she may have seen bc it only pits one sister against another and will do more harm than good--but let her know you're there if she needs to talk. Then talk with her about sex, bc, stds, love, emotions, boys, etc.
Talk with your 16-yr old and lay down future ground rules, no being home alone with her bf, no bf in her bedroom, etc. Don't bother rehashing the previous event bc it's unlikely she will admit to it, it will alienate her further, and what's done is done bc you implicitly allowed it by virtue of not directly disallowing it in her mind.
Then talk with her about her future behaviors, love, sex, teen sex, bc, stds, respect, emotions, gyne visits, etc. Encourage her to make emotionally and physically smart sexual decisions. Encourage her to wait til she is old enough to handle the responsibility and consequences of being sexually active, and if she has already started having sex, remind her that she can always pull back-- saying yes in the past doesn't mean she has to continue saying yes in the future!!!
Then have an open, unaccusing convo about her going on the pill or getting the depo shot if she is going to be having sex. Because if she is, she is. You will not be able to convince her not to-- don't even take that gamble. Give her literature to look over on her own time. Hopefully, she will not refuse birth control if she is indeed sexually active. Frame it as though she doesn't have to admit to having sex if she isn't comfortable doing so, but is she comfortable going on the pill "just in case" (because she prob is having sex already). Either way give her a bag of condoms to keep in her room, on her person, in the car, etc.!! Teens need more access to this sort of thing. Not providing access does not prevent them from having sex!!!!!!!!!!!!!!!!!!!!
Then take her to her gyne and have the the doc talk with her and also allow the doc to spend a little time without you in the room so your daughter can listen to doc and ask questions if need be without feeling uncomfortable with mom in the room. Have doc do a check up, test for stds, etc.
Then periodically have conversations with both girls together (it takes some of the pressure off of a one-on-one convo) or separately and remind them you are there to talk and that they can come to if needed for anything.
When I was in high school, by the end of Junior year, I would say 6 out of 10 people were having sex. In my peer group specifically, which was very social, went to parties, and was generally the "in" group, nearly everyone had had sex by senior year. They were just regular high school kids and nearly everyone was having sex, including oral. It's going to happen one way or another if the teen wants it to happen. Prepare her the best way you can. Communicate.
Best of luck!!