Do "Sisters in Law" have authority to order the couple to do something?

I am watching Sweet 18, and I noticed that the sister in law seems to have some sort of authority ordering Jung Sook around. I could understand this coming from parents or grandparents, but something does not feel right about a sibling doing this. 

I bring this up because I have seen this in other dramas. An example is "Our Glamorous Time" where it seems that the female lead's brother has authority over the leads, an they need to convince him before they can do some things.

So the question is, can a sibling or should a sibling have authority over a couple?

I dont know about Korea necessary, but it seems their annoying in-laws are simmilar to where I'm from. If you're a girl and unlucky enough, you might want to please your in-laws (mainly the female ones), so like the mother, sister, and grandmother inlaw. For the boy its the same, but for the males. So if you're a dude, you try to please the father, grandfather and brother in law. Usually tho, it's the girl that suffers, because in these situations, they guys are usually busy with work, and can't do much to stop their families or to please their in-laws since they aren't there. Thsi isn't as common as it was in the past, since there are differences in understandings and stuff, but its not unexistant