You are the brother to women - Women do not think like you do - never have, never will.
I have observed the female species (usually from afar - as far as possible) and note that they are real good at hiding problems when they want to.
Sure the guy looks great, is charming and smart and funny and whatever else you THINK are positives.
However its been my experience and the experience of people I have known, that the charming, funny, 'perfect' people usually end up being the ugly ones. For instance my 2nd partner was the life of the party, everyone just loved him. He knew all the right words, could pour on the charm and came off as a really good guy.
Until 11 months later when he decided to beat the crap out of me....
This is a thought, and only a thought. Your sister doesn't trust this fellow. Period. Women have that intuition thing, a 'gut instinct' about people and when they ask someone to step in and do them a favor like she asked you, take it as a symptom of something troubles her about this fellow, and she doesn't want to get into the argument of 'Its just a feeling, be more reasonable' that will follow.
And yeah, going out to drinks or whatever with him is kinda troublesome for straight people to accept as being innocent.
Oh wait, in this case it wasn't all innocent, the guy did things for ya, rocked the boat, got the juices flowing - whatever - your little head did some of the thinking when it came to 'drink or not to drink' with this fella. Be honest - you may not have intentionally acted upon these 'hidden desires' but don't you think that a little bit of that lusting thing may have compelled you to go against better reasoning that night?
I'm not saying you intentionally tried to step in her territory, nor am I thinking you would cross that trust barrier - however the mind plays tricks with us, and when we get chemicals flowing in our bodies that affect the brain (lust, love, hate, blah blah blah) our reasoning does get a bit bent, if not downright short-circuited at times.
I personally would suggest (Strongly) that you don't let the sister and others know that this guy made you 'happy' in a carnal way. I would suggest telling her you just wanted to get to know the guy and see if he is safe or not for her (Brothers do that for sisters so that ain't to unreasonable).
IF you approach it that way, maybe sister will tell you what is really going on in her head and heart over this fella. Your concern for her well being once expressed may jar her sufficiently to tell you what she is feeling - intuition-wise - when it comes to this fellow.
Because you are right, on the rational, logical level here her actions make no sense. There is something else going on, which most likely does defy logic and reason. And never - ever - make the mistake of dissing a woman's intuition. Not only does it end badly for you in that it will piss her off (Hell having no fury.... etc.) it also has a strong possibly of leading her down the wrong path.
Intuition is not a magical, mythological beast. It is one of several ways that the brain uses all the data at its disposal (And humans collect a hella tons of data that they are not conscious of) to reach conclusions.
Clearly wanting you to lie for her was the 'better' choice in her mind. Since you are 'a dude' and if that avatar is you, you are an imposing dude - which means you are a potential protector that carries a bit of measure. Clearly she felt a man's intervention was needed - again it may be intuition at work.
You need to get to the point with her where you two can actually talk about stuff like this and her not worry that you will dismiss her 'intuition' or 'gut feelings' or what ever. I don't know your relationship with her, but if you can recall a time she shared this 'sixth sense' with you and you dissed her then she may be most reluctant to discuss. If its others in her life that did that, then again she will be reluctant to discuss.
And to cover a topic again, lastly to impress upon you the potentials here:
"
Men that are abuse are very clever, smart, and extremely charming. Most of these men have a personality that draws people in because of their level of charm this is part of their art to deceive and manipulate. This is why often times when a victim does report an assault she is not easily believed because people usually say “not him, he is so nice’ “you are so lucky”, All of this plays into his because if he gets people outside of the home to buy into his deceit the victim has little if no support."
Source:
https://www.psychologytoday.com/blog/the...-men-abuse
Now I hope I'm wrong about this fella - but you should read up more on the mindset of abusers - that link is a good start - Google for more.