I feel for your DSD I really do. She has no private space.
I feel for you OP. You have lost some of your private space and now been embarrassed, and i feel the situation would be very different if this were your own DC.
Personally, i don't care much if i was 'walked in on' by mine. Loo or bedroom doors as things of privacy became lost to me when the DC started arriving.
Idon't have any DSD but i do sense it would be different as they have a DM who perhaps isn't bothered about doors either. However, she is allowing her DD to stay in another's home where she is suit in suitable sleeping arrangements. How has this come about?
and the person who should be pivotal here and managing this properly seems to be absent from the situation? He has presumably 'wanted' his DD to sleep over in unsuitable arrangments too? What message is it giving to his DD to not provide her with space and expect others to all go without their privacy too and have a sense of not 'being allowed' to speak out easily about this?
Its clear that DSD should not be sharing with your DS, or you. if you are not both very comfortable with sharing a private space then its not working. Ordinarily walking in on a DM is no biggey, DSD seems very comfortable and not considered the privacy issue, with no idea that you would instantly have whipped off your panties, and you live in fear of her DM.
So there are many issues going on here. They all revolve around your DP and you already alluded to this.
Any agency involved would not allow his DD there in these circumstances. She doesn't need to stay over, she can be there all days at the weekend. She is pre/pubescent and is very old for this situation. Why is she not living with her DM and spending time over with DF as and when she wants? Doesn't sound like anyone's happy, neither should they have to be and DP's finding it easy to ignore it all.
Everyone does seem to be fitting in around DP/DF.
How on earth did an arrangement come into being with two opposite sex siblings over 4/5 have to share the same living space? its wholly wrong and everyone is going to suffer for it.
I don't believe you 'have' to share your body with them at all, and yes, do have your privacy and teach them boundaries, but how is that possible in practice when they are being given none?
Its the set up thats wrong. I think you've not done anything wrong per se, but that you can't even feel comfortable to speak to her about simply knocking a door speaks volumes really, somehow you are walking on eggshells around this whole situation.
Both those DC need their own space. Have the DSD one week and the DSD the other, then everyone has their own space and privacy.
I went through a similar situation, but courts and outside agencies involved. the DF was trying to force DD to sleep in his gf's DS room, but i was told clearly this will not happen as it infringes the DCs rights, both of them. His alternative was to have her sleep in a communal area whilst everyone else had privacy, both options are wrong and tell the DC their needs are not important and that privacy doesn't matter, and as everyone has acknowledged on here, it really does!