Thats not a favour, thats a fecking quest

I think the fairest way to do it, is that she pays you what you would of earned that week so your not loosing out. You cant exactly tell your mortgage or gas/electric man that cause your such a nice guy and fitted your Sis's kitchen for free that you'd like to miss the payments for that week .....

If she doesnt agree to it, then thats just blatantly cheap skating with a rocket on both boots!

But if you dont mind doing it for nothing, then go fo it