You may be on to something. I hope they treated both rooms like a crime scene to preserve potential evidence but I fear you are correct, that the couple's drink containers are gone.
The alcoholic drink was indeed from the mini bar...
Pennsylvania Woman Died at Same Dominican Republic Hotel 5 Days Before Engaged Couple Was Found Dead
The above article also states
they had just checked into the hotel. So it does sound like whatever she drank in that minibar was contaminated, whether purposely or not. Either that, or the timing is just a coincidence. Article said she took a selfie soon before dying... Usually people don't take selfies when they're feeling gravely ill, so I assume she was feeling fine at that point. Whatever happened, it killed her very quickly.
ETA quote from above article:
"As they were getting situated in their hotel room, McDonald said
Miranda 'decided to relax with a drink from the minibar' and mixed a bottle of alcohol with a soda. But
less than an hour later, things took a turn for the worst.
"'Suddenly, she called out to Dan and he came right over and
she was unable to breathe. She collapsed, she couldn’t communicate,' he recalled to Fox News. 'He did his best to try and resuscitate, to help her'...
“'It was very sudden,' he noted. '[Daniel] said
one moment [Miranda was] taking pictures, smiling, happy and the next moment, in extreme pain and collapsing… she had no known health issues that I knew of.'"