God yeah - my insecure ex never learned to trust me after 25 years during which I did nothing to give him cause not to trust me. He would get paranoid about what I might do if I got "pissed" (I never drink to excess), though conversely he would encourage me to drink rather more than I wanted to at home.
If I'd realised, the first time he kicked off, that it wasn't normal I would either have shown him the door or stepped on it firmly, instead of wasting years trying to be reassuring and demonstrating my trustworthiness. Because he had what he himself used to jokingly call "a worm in the brain" which would never let him relax and trust me. Or perhaps because he was a controlling arse. I'll never be completely sure which. And it made life bloody uncomfortable, let me tell you. I hardly ever went out for drinks as such (once a month in a good year), but I went to work every day, the shops, occasionally an exercise class... all times I was out of his sight so who knows what I could have been doing? Heaven help me if I was 20 minutes late due to my train being cancelled.
So, with the benefit of hindsight, what would I do if I were the OP's sister? I'd tell him that I had a nice night out with my friends, thank him very much for asking, and if it upsets him that is his problem. If he does leave over such a minor matter he was never hers, as the old saying goes. On the other hand if he genuinely cares about her he will take a long hard look at his own behaviour and decide that having her in his life is worth a few compromises - on his side as well as hers.
When one partner has to make all the changes and do all the forgiving, that's not a relationship, it's a dictatorship.