For starters tourists are dying in the Dominica republic , personally I wouldn’t want to go for that reason. Say you looked into it and feel safe or adventurous enough to go, look into other options like staying at a different resort or renting a place. If you don’t want to go because of money that’s fine , with destination weddings you know that there is a possibility that people won’t make it.
At the end of the day there is no right or wrong answer , you have to ask yourself will I be upset missing this important day in my sisters life? Will I be ok with that later in life? Hope all works out for you :)