I'm going to check up on my ex boyfriend at the request of his sister against my current boyfriend's wishes in the middle of the night
Why do you have to go through such lengths for your ex?
I want to stop his sister from telling lies.
It's nice you just want to try to help, but an ex and their family is an ex no matter what. You should prioritize your current relationship and explain the full situation to your boyfriend.
Maybe I'm just going to see what he's up to and that is it. Maybe I can still help them.
If checking on your ex will be disrespectful to your current boyfriend, you have to weigh which relationship is more important to you. You may damage your current relationship by getting involved with your ex on any level.