Re: In love with enemy
Tipu, if you are unable to convince your daughter then clearly there is no choice left but to play along her wishes whilst keeping an eagles eye on that family's behaviour and actions.
The thing is , your daughter might rebel immensely if you say a direct no to her. However if you do give it an honest try and supposing she gets let down by the guy or his family , during the proposal process then at least she will come back to you and you wont lose her completely.
As a good father you are doing the right thing suspecting that family's intentions, please try to meet with the guy yourself and try to find out how he will go about convincing his family. As namaan said above, at the end of the day , they are family and this incident might only join everyone in happiness , if Allah wills it.
Otherwise , your best weapon is to deal with the guy directly and make sure you keep your daughter upto date with it all. If you suspect then that they are cooking an evil plan , there are chances your daughter will be able to see through it ,with your help.
Good luck.