Have i done anything wrong?

Last night i went home after my weekly kickboxing game and i was a winner last night so i went home with lots of prizes and stuff...and my sis was really delighted and she watched the video where i was fighting and winning the game...she was so delighted that she hugged and kissed me and later on she put her feet on my lap and asked me to give her a foot mage...for which i was a little hesitant as i myself am a female feet lover but i maged her feet while watching my video and to be true i got so hard that i don't know how to explain it as it was so rock hard for me that i came at the end of the mage but she enjoyed it really and asked me to mage her again....but i think i won't do it as it's really gonna take me to a point of no return...pls advise..tks...