Archana claimed that she gave Purvi acchi sanskar. I don't think Archana was able to teach Purvi much about family ties and value of marriage and family. First of all when Archana found out that both her daughters fell for the same guy she shud have taken care of things right then and there. I know she had no control on Ovi at that time, but she cud have explained to Purvi that no matter how good a guy Arjun is, no man is worth hurting your sister for.
If it was so important to hurt Ovi's feelings and get Purvi to marry Arjun, Archana shud have asked Purvi to wait a few months and give Ovi time to digest the situation, after all Ovi was suicidal at that time. No sanskari larki would get married while her sister is trying to kill herself due to rejection by her honewale dulha. But Purvi went on with her wedding plans happily and did not feel bad for her sister at all.
A sanskari larki does not decide her mother's fate at the age of 19 without consulting with a guardian or parent. When 2 inexperienced 19 year olds agree on a deal and decide the fate of their family without the knowledge of the elders this is what happens.
A sanskari larki does not insult her mother by not showing up at the mandap for wedding. Ovi has never claimed to be sanskari, rather everyone knew that she was a lunatic, so not much was expected from her anyways. If Manav and Archana gave their daughters the right parvarish then things would not have gone out of hand today. I think Manav and Archana are 100% at fault for whatever Ovi and Purvi did.