The real Chris and Heather can be seen dancing during the fundraiser scene. Just after the scene with main characters having their photos made, the character of Chris is walking toward Heather and you see a couple that he passes. The real life Chris has a shaved head wearing a blue shirt. Heather is blonde wearing a burgundy dress.
Based on a true story, with a wedding picture of the real Chris and Heather Dempsey shown in the end credits.
It is entirely possible for sisters to have different blood types.