What is Sileh Rahmi?

Any behaviour which is generally regarded by people as good, kind, gracious or obligatory, when directed towards your relatives is Sileh Rahmi.

Imam Ja'far as-Sadiq (a.s.) says:
"The kindness towards relative and goodness towards the believing brother makes easy your reckoning on the Day of Judgement and protects you from sins. So you should be kind to your relatives and be good to your believing brothers even if it is as little as a sincere 'salaam' (salutation) or a hearty reply to a salaam." 
(Al Kafi)

At another place the Sixth Imam (a.s.) remarks:
"Do Sileh Rahmi towards your relatives and the near ones even if it is just by offering a glass of water to them." 
(Wasaelush Shia)