Trying to masturbate but no privacy

So my sister and her boyfriend have been dating for about 8 months and stays the night all the time and he is always over and I have two other siblings and my mom and her boyfriend I never have privacy and can't ever get to the deed when I have an urge (my urges don't come often) and it leaves me irritated I've asked for them to stay at his house then I get asked a bunch of questions about why? I have almost had enough and don't know what to do I need help ASAP. Any advice?