Do not allow your partner to be left out of your family events. Your sister HAS to learn, he is your other half,whether she likes it or not. It simply is, so live with it.
If there is a family event and he isn't specifically invited, you don't go. You don't apologize, you don't explain-you just don't go. If he isn't welcome, you aren't welcome. When he is welcome, you'll be there.
For the first 15 years of our relationship, when my sister called our house and my partner answered she gave no acknowledgement of who he was, she simply asked for me. Then one day I realized how badly I was disrespecting him by allowing this bullshit to occur. It ended. Now she talks to him when he answers the phone-just like he's a real person. Just like I talk to her husband when he answers the phone.
My family went through all of this over the last twenty years-confused parents, angry siblings, nieces and nephews....Next Sunday my partner and I will drive three hours to a family reunion of my siblings who all love him. We'll eat, laugh, hold babies and enjoy each other. I never thought it could happen but my rural southern redneck family surprised me. Hope yours will too.