You certainly have the right to call social services yourself - but it is unlikely to improve relations with your sister-in-law.
From your background I'm sure you know that these are often really rough situations where everyone involved tends to not act like "themselves" because of grief and anxiety. You might be able to help your sister-in-law and your brother more than you realize if you can build a little trust. Good luck.
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Not unless your brother has given his authorization for you to act in this capacity. Generally, it is the spouse that gets to make the call.
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