going on a butlins hen do pregnant???

Please help ... I have my sister in laws hen do when I will be 20 weeks pregnant it's at butlins for an adult weekend there is 12 of us going and I arranged it all before I found out I was pregnant, someone else has asked to come and paid the full amount but now there is not enough room for the extra person as there is no more rooms ..... so question is do I drop out or do I go I have not told my sister I'm pregnant yet and don't know how to either as I am also 1 of her bridesmaids 🙈🙈🙈🙈