OH yeah I missed out one point if the Nikkah happens and they have sex and then for whatever reason the wedding does not happen (this does happen) then the girl is screwed up coz she had no formal thing meaning no witnesses apart from the two or three required for the Nikaah so before you all start having sex and stuff think through what you’re doing.
Once the nikah is done, wedding HAS happend. So there is no sin if they have sex after nikah only. Now the social consequences of their relationship becoming public before ruksati is a whole other issue. Still, it does not change the fact that the couple are halal for each other after nikah. They can do anything that is lawful for any married couple, period.