just go ahead with the travel.
the idea you have to be clear is that your bf is yours, not hers,
if she fared up with you, then ask her why.
she should respect you as her senior, you give her the respect a
couple of times, if this time, she still behaves the same as per
last time, fared up without giving any reasons, don't care what
your sister feels.
sometimes in life, you have to know when to be selfish and when not
to be selfish. 
this is your life, and probably your husband to be, she should be
happy for you too, talk to her, if she respect you, then she will
tell you, if not just go ahead with your plans on travelling
then.
