My sister in law is allowing her daughter to live with the man who molested her (sister in law) as a child through teenager in the same trailer park as her own (my sister in law's) brother who is a convicted child molester also. I feel like this is endangerment to her daughter and neglet also. Her younger daughters reside with their father and are indeed safe but what action can I take to protect my 14 year old niece from her own child molester grandfather, considering her mom put her there?

First off, when using the legal forums, you question needs to include your location, as laws will vary. There are a lot of details that are unclear from your question. Does the child live with the accused or just in the same community? What are the conditions of his release and registration, if any? Has there been a conviction? Is he on the sex offender registry, or is the abuse just known within the family? If they simply live in the same area and he was never tried/convicted, I don't see a whole lot that can be done.

Grandfather or uncle? Not that it matters legally.

Was there a conviction?

Was there a police report of the molesting, was there a conviction? Is the man a registered sex offender (depending on what country)

If there is proof of this abuse, and depending on where this is, you notify Children's services