U can board. The same situation happened with me and my brother while returning from musourie exp this April. My brother tkt was in rac and mine wl1 on same pnr. After charting the tkt was confirmed partially with my brother getting rac with other passenger and me still with wl1 in 3A. We boarded the train and tte didn't utter a single word. I told him abt the situation and he said let me check first. After lrj jn he gave us the UB as the 3rd passenger was a girl in mid 20s and the courtesy demanded so. However after few mins tte came again and alloted me with the berth as some passenger had not boarded. Don't worry. If a seat is vavant it will be provided 2 u