How to stop someone from taking undue advantage of my dads property

My sister and her husband is taking undew advantage of my dads property. Her husband and she was almost enemployed because of there own faults. He is leaving jobs frequently. Also she managed to keep his only son in my old aged moms custody from childhood deapite her motherinlaw stays in the same city she is not giving her son to her and her husband is also not opposing. And freqquently taking financial backups from my dad. Dad has also givien 14 lacs for purchase to flat in ahmedabad city. I dought emotionally my dad going to loose all his hard earned money behind them. Im the only son im not interested in my dads property and in the same time i dont want that some taking undew advantage of my dads hard earnings.