Before Clary saw Jace (for the first time after they find out that they aren't siblings), she's talking to Aline and Isabelle about Jace, I think, and Aline says something like "Maybe he's not interested any আরো because it's not "forbidden" any more", because Jace hasn't turned up for the party yet.
When they see each other, Clary tells Jace what Aline said, and Jace denies it, assuring Clary that he loves her no matter what (I think it's this line: "I প্রণয় you, and I will প্রণয় আপনি until I die, and if there’s a life after that, I’ll প্রণয় আপনি then.") And then he says "But আপনি can forbid me from doing this", and kisses her.
Meaning, if they still want to keep the "forbidden love" thing, Clary can "forbid" Jace to চুম্বন her (as a joke) - so it would keep that element of "forbidden love"- but without the "it's-incest-because-we're-siblings" thing.
আপনি mean at the end where they are at the steps and Clary asks if they will still প্রণয় each other now that they are not 'siblings anymore' and it wont be forbidden, correct?
Well.. To me it sounds like they might have sex. But then in a Q&A with Cassandra Clare, someone asked if Jace and Clary had sex on the steps and she answered something like this: "That is for আপনি to decide. But do আপনি really think that their first time would be on some cold steps where the whole city could possibly see them?"
umm i dont know but it made me mad at the end of city of ashes when clare goes to tell jace she's ready to be with him and thats when he desides to start being her older brother!!!!! i was all nooooo!!!! i found myself screaming at the book all "how could u do this to me" I have to go buy the third book quickly!!!!!
he's basically asking whether clary's still interested - he's worried she might not like him anymore - what a fool! and he's saying "if you're not interested, then tell me not to চুম্বন আপনি and i'll get it" i প্রণয় him!