If the husband’s brother came to a house in the medieval age, I assume he could get room and board for a night. Would the wife’s brother get the same treatment?
to hear some people tell it, back in the old days everyone could get room and board for the night in anyone’s house… only now do we stoop to the level of selfishness as to deny someone compassion
not that this helps answer your question in anyway… i’m just a bastard like that
Is this another thread on incest? or time travel? 
or both… time-travel incest… now THAT’S kinky!
The Medeival Age covers a lot of time and a lot of geography. Customs and practices differed quite a bit from region to region. Are you interested in any particular region or time period? You should search on terms like “medieval etiquette” or “medieval customs”. Also, check out the voluminous Bracton’s Laws and Customs of 13th Century England. It has a search function which should help immensely.
I’m not an expert on medieval customs, but casual reading suggests that guests would not be turned away, particularly those with ties to the household. In any case, there were a lot of rules. A wife’s brother might be seated to the right of the head of household along with other guests in order of wealth or status.
I think that manners and customs from times past reflect more the behaviour of the ruling and powerful classes rather than the vast majority of the population.
In 13thC England, for instance, movement for peasant tenants was very limited in the countryside.
In Georgain and Victorian England the so-called correct manner of behaving, as published in the then fairly recently economically available books, was more concerned with the affairs of the upper and middle classes, and are partly satirised in Pride and Prejudice, and many of the Bronte novels etc.
All I am saying is that you have to look at which layer of society you are interested in, those at the bottom of the pile such as serfs had a whole range of differant obligations to those further up it.
I was going to write a novel where a man and woman through time travel become their own father/brother/son - mother/sister/daughter. But after trying to sort out the paradox loops, I had to lie down in a darkened room with a cold pack over my eyes for several hours.
The short answer is, it depends on the relationship between the wife’s brother and the family ("I don’t care if I married your sister, you still stole my cow!), where the people in question live, (“No, you can’t sleep here…you live next door”) space, (“It’s not that we don’t WANT you here, but this is a one room hut, and we have 20 people sleeping here as it is”), and all those other factors which influence human relationships.
Take a look at Heinlein’s story “All you Zombies”. It makes the paradoxes clear enough that you won’t need the dark room and cold pack.
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