Would you be upset if your boyfriend smacked your sisters butt ???!!

So basically there has been a time where my blood sister had had allegations from her previous boyfriend about her and my boyfriend doing something .

i kind of by based it and let it go because they denied it and i didnt really believe they would do something like that but now i been seeing some Things are making me feel uncomfortable with them two I have been letting her stay with me for the past year because she lost her place so me and my boyfriend is a given that we had been talking about doing YouTube and funny skits so the other day he had called me and her to the living room and knocked her but he just called me over and just did a look im like what he didnt say anything he looks back and fourth at me and my sister i thought he was gonna say something dumb so i start laughing and said what ? my sister was like u play to much and turns around and start walking away then he just without saying anything smacks my sisters butt and then my sister turns around and was like was that you sis i was like no it was e**** my boyfriends name i start looking around after he did it in confusion and he just starts laughing and was like it was just a joke and my sister just laughs to and walks away saying what the hell and previously we were all just talking about how my sister doesn't have a boyfriend and she can't find anybody and my boyfriend and I also said what is we all just be together and we all laughed it off so when I asked him pull him to the side like why did u smack her butt his reasons where remember when we were talking about she needs a boyfriend i was trying to play a joke on you joke we should all be together and remeber when i told you about youtube i was gonna record it to be funny skit i just didnt have time to put the camera on . I'm thinking one that he didn't have a camera insight to you never even mentioned it to me like so we could be in at together so I'm aware instead of just doing something like that without telling me first and running if I need to make sure that I feel comfortable. three there were allegations said before so I don't even feel like you should even be doing something like that I told him . what do you guys think . am i over reacting ? jelousy?