firstly rishi knows how much indira loves her family...secondly indar is indira's father.( worst father ever )...so even if indar would have been the thief than indira will be called the daughter of the thief...and khandan ki badnaami to hoti...like jab munna jail gaya tha...he saved him too...he just dont want any badnami for the sharma's ...not in any way...as already indira had suffered enough...and he wasn't telling indira the truth in police station...because it would have braked her...it did break her...her father did this to her...what could have been more worse than that ?????..Abhi to she dont know that even mother was involved...for rishi...indira already was hurt because of her family members previous response...when she was running to them and asking them...that don't they know her ????...can she be a thief ???? and said to him...that she never got love of the family members...thus he was hesitating in revealing...he knew it will hurt even more...i mean kahi na kahi indira did started developing the soft side for him...that was the reason she gave money for his shop...rishi thought he will handle mr sharma on his own and will keep an eye on him in future...
so the only reason for not getting indar arrested was because he was thinking about indira...and the pain she will went through after listening the truth...