This is an article describing a problem I have with the story-line in the finale of season 5, The Gift. In it, Dawn's blood has been spilled in a ritual which has opened a portal to multiple Hell dimensions, and her blood needs to stop flowing in order to close the portal. Buffy remembers a moment earlier in the season where she'd described them both as sharing the same blood, so she saves her sister by sacrificing herself instead. It's a moment with huge emotional impact but it doesn't actually work logically.
The first issue is that shared Summers blood only works in a metaphoric way. Buffy and Dawn don't literally have the same blood. They'd need to be identical twins for the DNA in their blood cells to be the same, which obviously they aren't. And even if they were genetically identical, environmental factors such as what they'd eaten recently, how active they'd been and their recent health would cause differences. Buffy being the Slayer while Dawn isn't must also make their blood different, given the occasions we've seen where Slayer blood has special significance.
The second issue is that Buffy's death didn't actually stop Dawn's blood from flowing. If anything, the idea of shared blood would mean that both sisters would have to die to close the portal. For this reason, the alternate solution of a vampire ally such as Spike or Angel drinking her blood, seen in various fan fiction stories, wouldn't work either.
As I said above, Buffy's sacrifice succeeds in terms of emotional story-telling, which of course was its real purpose. It was unpredictable given how the audience had been led to believe that she might have to kill Dawn, and it was a good development from the previous time where Buffy had been in a position of needing to sacrifice a loved one back in season 2. It'd also have made a good ending for the series if it hadn't been picked up by another network. But within the actual story it doesn't make sense.