No, you can't sell them, as you don't have legal ownership. You do however have a tortious claim in nuisance and trespass to land, and could seek an enforcement of specific performance by the courts. This is an equitable remedy, so make sure in everything you do up to the case is fair and honest or you may be denied the remedy. You could also seek damages but this would be limited as you have suffered no harm or financial detriment. Try threatening to have the vehicles moved and pass the cost of removal on to your relative. Hope this answers your question.