Dear Harshal,
This is a Very sensitive issue, one is both the parties ( Organisation and your Sisters Family) need to be flexible and come to a win win situation. I am sure in this tussle you sister would be getting stressed out, which is not right as she wont be able to give 100% at both the fields.
The Family should take step where instead of traditional Magalsutra they should allow new simple designer mangalsutra , today we get variety of Mangalsutra , which more of like a chain and not the traditional design.It would look like a Mangalsutra and same time look like a chain.
The Organisation also to take one step ahead where they speak to her family and try convincing them. also give your sister the liberty to wear a magalsutra which dosn't look like a mangalsutra. Also they should make it clear during the hiring process that mangalsutra wont be allowed.
Its easy to get employees, but difficult to get dedicated employees.