What if I get on my sister's fiance's instagram and mess with her?
What is the reason that you want to bother your sister?
I hate my sister. I planned to show her that this guy's creepy
You should not interfere in your sister's relationship by using her boyfriend's social media account. If you do this, you will bring unnecessary tension and trouble to your sister. This will cause your sister and you to have problems.
Whatever. I'm 100% sure that he is a middle aged man pretending to be a 20-something. This will force her to see that and she'll break up with him.
It sounds like they love each other enough to get married and you may ruin it if you go through with this prank. People would think that you're the reason they broke up and hate you for it.