Subsidiary & associate companies

A company is siad to be subsidary when it is controlled by parent i.e when it holds 51% of subsidarys capital (both equity and preference put to getther).A concern will be an associate when it holds significant influence ie.20% and more than 20%.Significant influence means power to control the operating and Financial policies

I dont Know what is sister concern I think Companies within the group is termed as such.

About Associates You can Refer AS-23 and AS-18

A sister concern means an concern which comes under the same management but there is no control amongst the companies in one another.  For instance Tata motors & Tata Steel are sister concerns as they are under the same maangement but do not have any stake/control between them.

What Sivaram has said about subsidiary & associate companies I agree with him.