Is there a reliable neighbour who would be prepared to undertake this task for a small fee? Anybody who lives within sight of the house would be better placed to check on its security and your sister in law might feel happier with somebody she knows looking after the place.
As regards forwarding mail, why not suggest to your your sister-in-law that she makes arrangements direct with the Post Office for mail to be redirected for a certain period of time, for payment of a fee. (This is what people normally do when they move house, but the Post Office can almost certainly do it for temporary period too). This will probably be far more convenient than expecting somebody to continually check for mail and have the bother of re-addressing it and forwarding it on, especially if the mail contained some urgent communication.). Assume that in the meantime your s-i-l will arrange for all her utility bills & credit cards bills to be paid by direct debit so that her financial commitments remain in order.