There might be two reasons why there was no immune response:

• The immune system responds to protein on cell wall known as Major Histocompatibility Complex. Siblings share many if not all of the important MHC proteins to seem identical to immune system. Maternal twins or identical twins share all of the MHCs. That is one of the reason why there is a high chance of you getting a compatible tissue or organ donation from siblings than parents.
• Sperms and seminal vessels are essentially outside the immune system just like spinal disc, eye lens, joint cartilages. Immune system doesn’t even know they are present.