Nancy has a boyfriend (Ned) and one of the Hardy Boys also has a girlfriend I beleive. But if you stick with the origianl sereis and not the newer “case files” series you should be pretty ok. If you can stomach them, perhaps read them first.
The Bobbsie Twins are kids so those should also be good – but I don’t think they involve murder much.
Of course Sherlock Holmes is the quitessential detective.