Wat do u ladies think? My boyfriend& my sister????

Me and my boyfriend got into a argument one day and we was both hitting low , and he said to me " thats why i fucked yo sister ! My boyfreind is good at Hitting u where it hurts..so after our argument he apologized and let me know that he was just tryna piss me off... so now i cant shake the idea that something went down between him & her.. my sister is 5 years younger , she also the jealous type,She have done weird things to me in front of my boyfriend, like showing off in front of him...getting smart with me... talking to me like im a child...etc.. but its only been one time that they were ever alone together.... i.was getting off work she was coming over tht day and she made it to my house before me, my boyfriend was also there... other than that i never left them alone. I love my boyfriend with all my heart, but wat he said is lingering in my mind.. im so confused rn... and the thought of it makes me cringe...