Sperm heads can survive a wash cycle and DNA can be extracted so innocent transfer can occur. Sandra Lean has explained this in her book so she is not trying to hide these facts.
Testing for innocent transfer in this way was not available back in 2003 so without this being tested it is just a theory, although plausible. However it was not just the sister's boyfriends sperm which was found on the t-shirt, there was multiple sperm found which was not attributed to anyone – not sure how this can be explained away. Not forgetting there was no DNA belonging to Luke.
Also what is clearly explained through evidence is that there were multiple black t-shirts available for Jodi to wear. Why would she wear one with visible staining on it, when other, identical t-shirts were available?
Now that Luke's case is being re-investigated perhaps all this will be explained once and for all, but without the testing having been done at this stage whatever is said is just theory.
This still does not give an explanation for the 2 people who were present at the crime scene at the exact time the murder was supposed to be taking place.