Kissingg for the first time?

me and my boyfriend have been going out for 6 months now & ireallyy want to kiss him. he has told me he wants to kiss me to, and when he was going to i ruined the moment cause i had to go home suddenly and he couldn't try in time. he is coming to my dad's party tomorrow at the firehouse my dad belongs to, and i want to kiss him tomorrow. the only problem is my sister will be there. is there any way we can get alone cause after we do it will deffinatly happen. but i am on crutches cause of a sprained knee, and i am a little taller than him to begin withh ( he is about 4'11-5' and i am about 5'4) any ideas? pleasee helpp <33 thnks sooo muchh(: