should I be a good little sister and help big sis with her relationship

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should I be a good little sister and help big sis with her relationship or not? Sometimes I get to be involved even if I dont want to so what should I do? Sorry in advance for long post

Problem: we live together and her bf moved in. Later in winter or spring they will move to his flat (now under reconstruction). There hasnt been a day that they wouldnt fight... She comes home tired mentally/emotionaly stressed, he comes not so tired but his work invlolves physical strenght, so he cant understand why she comes home so tired... Another thing is money: he gets almost twice as her, but spends most for his flat reconstruction... There seems to be tension about money issue... Also dividing home tasks: she makes food and other chores, he washes dishes only...
There is clear unequality between how they manage their everyday issues, but most important thing is that they dont want to let go of their hurt pride? Like even though some things are to be forgotten but they both bring them up again... And they BOTH do this...
My sis doesnt listen to me sometimes and I'm not that close to her bf to give him advice, they are also 27 and 28, while me is 23 single...