Well now that I've made it pretty clear as to what yuvi's intentions are behind returning and marrying mahi. First of all twinkle and kunj could call the cops to arrest yuvi as they are still looking for him in relation to the kidnapping and attempt to murder of both twinkle and kunj, but he ran out of the country to escape arrest. However I know this will not help as mahi, who suffers from severe anxiety and low-self esteem issues will be emotionally broken if yuvi is arrested and she may try to self harm as they showed how mahi ran away from the house and ran on the road almost getting herself killed in the process so she is capable of hurting herself for yuvi. The family will evidently be fearful of mahi hurting herself if yuvi is taken from her so the family will have no choice but to drop the charges on him in all probability.
Now coming on to what I think they should do is something that perhaps is actually very simple but I know these stupid CV's will not pick up on this idea. Well what I suggest is that twinkle needs to realise herself and make her parents and kunj realise that yuvi is using mahi as a pawn against them to emotionally blackmail them as he feels he can force them to do as he wishes if he threatens mahi's life or happiness, so to combat this they need to react completely opposite to how yuvi would have expected.
What this means is that twinkle should tell her family that they should pretend to break off relations with mahi in front of yuvi, mahi and everyone else and tell mahi that she believes her husband more than her family and went against them for him so she should go with him and they will not have any relation with her and they will ignore her from now on. They further need to tell mahi in front of yuvi that if anything goes wrong in her relationship with yuvi then it will solely be her responsibility and her fault and she will have to deal with it all herself and not expect a shoulder to cry on from her family who she ignored for a guy she only knew for a few days.
This act needs to be convincing for yuvi to believe that the taneja's have broken relations with mahi and his plan of emotionally blackmailing twinkle using her sister has flopped as they will not come for her rescue as they will make him believe that they are angry at mahi for ignoring her families pleas and advice for her. He will be furious realising this and may even leave mahi or start showing mahi his real nature when he starts to think she is of no use to him.