Have u continued haveing sex when some one walks in?

-------------- this one time i was fucking my GF and her younger sister [12yo] walk in on us and i was just about ready to cum so i did not stop doing it till i cum my GF did not know she was there till i roll off her and my dick was still hard her sister said something to her then my GF cover me up fast she never knew her sister was there for about 5 min. watching us fucking