Hi jenny01,
I'm so sorry to hear that it is hard for you to find privacy in your house, I can imagine that could be very difficult. Because I don't know you or your family's schedule, I can't tell you when would be the best time to masturbate, but I agree with Mo that it is important to set boundaries about masturbating when others are present. I know you said you don't like to masturbate in the shower, but could you try masturbating while you are in the bathroom and out of the shower with the shower running? (Not the best for water use, but it could be an option!)
I also found this article about masturbating without much privacy -
https://www.scarleteen.com/article/sexu ... rbating_at
One of the points that it makes is that you hear your voice (and any noises) the loudest, and that most softer noises can be covered by music, fans, running water, etc. The article also suggests that you can prepare "excuses" for alone time (needing a nap, not feeling well) when you know your sister won't be in the room.