I have a 13 month old boxer cross and when we visit other peoples homes she poos and urintes on their beds. She knows she has to go outside to do it but in my mums house especially she goes upstairs and poos on my mums bed and my sisters she has done it on my mother-in-laws bed too aswel as my 8 year old sons bed at home she knows its wrong and she knows she has to go outside but sometimes she will choose someone's bed instead can someone please tell me why she is doing this and what we could do to stop it?

It sounds like it may be a dominance issue.
She poops and pees on other peoples beds to mark them as her own.

Has she been spayed? That might help the problem even though it is usually males that exhibit this behaviour.

You could try putting some baking foil on the beds to stop her jumping up on them. Usually when they feel or even hear to foil under there feet they will immediately jump off.

Yes it does look messy but it would only be until she learns not to jump up on the beds.

No she hasn't been spayed yet. Thank you for your answer.