Wow. MASSIVE alarm bells are ringing here.
He told you his ex's sister would be staying while you were away. Presumably that was ok with you.
Then he adds, after you've left, that actually his ex will be staying too. Not cool that he left that out initially.
You ask him directly the next day if they shared a bed. He says they did. Ok, it's good that he was honest, and arguably that suggests nothing happened, as if so it would have been easier/likelier to admit nothing, knowing how it would sound. BUT he was only honest when you asked him straight, and you've said you gave him opportunity beforehand to mention the sleeping arrangements.
At worst, he and his ex had sex in the bed you share with him.
At best, he is an inconsiderate, thoughtless idiot. I can't fathom how a guy would genuinely believe his girlfriend would not have a problem with him sharing a bed with his ex, especially after he's not been forthright about the situation.
So he doesn't come out of this very well either way.
I am tempted to tell you to dump him, but as someone else said it's much easier to say that to a faceless internet person than to do it if it were you in the same situation.
It would probably be best to sit him down when you're back, put all your cards on the table and be honest about how it made you feel, and ask him to tell you if anything happened.
If he says that nothing sexual happened, and you believe and trust him, then by all means stay with him, but tell him that there can never be a repeat performance as you are not comfortable with it and he needs to put you first.
If you don't quite believe him/still have doubts, you probably always will, and it would be best to cut your losses and leave now.
All the best, OP.