Walked-in on Head? 😰

Summer
So my boyfriend and I had had a college overnight at two different colleges and I picked him up the next morning. The both of us are feeling horny so we get to his house and we're in his living room. His sister is home too and she asked us to go upstairs because her boyfriend was coming over. So we thought cool they'll leave and we get to go upstairs. 
So we ended up having sex and afterwards he decided to give me head. Then his sister walks in and says "REALLY?!" 
I've never been so embarrassed! 
She's 15, and idk...
have you guys been walked in on during sex?