1. Brad Pitt and Benicio Del Toro: As hunky brothers
2. Rachel Weisz, Preity Zinta, Vyjantimala: Vyjanti-ji being mother
3. Helen Hunt and Leelee Sobieski: Mother-daughter. (They're so similar it's creepy.)
4. Jack Black, Philip Seymour Hoffman: Brothers
5. Diane Lane and Jennifer Lopez: Seriously. Sisters.
Or cousins. Or aunt-niece, depending on makeup.
6. Robert Downey Jr and Javier Bardem: Be still my beating heart. Brothers.
7. Nicole Kidman and Naomi Watts: They do look like one another, they do.

(Nicole has to stop the botox. She'll start looking like Renee Zellweger soon, the way the both of them are going.)
I'm pretty sure there's many more I can't remember just yet. It will come to me. In the meanwhile, please feel free to suggest.