I say IMHO, do what gives you peace of mind, my father in law really despite me, leave my hand in the air when I salute him, never say hi, turn his back on me, only when he need a favor he call me, one time I was invited to eat at his place and I park hindering the drive way, I ask the old man the keys of his truck so I can park correctly and I say to him can you give me the keys of your truck so I can move, am hindering, and his answer was " for a long time" I say nothing , trow the keys back at him, call my wife and I told her how sick and tired I was from this ungrateful sob of her dad and never want see him again, she say fine is your right to do it so.
When he got very ill just Couldn't let my wife alone with all the burden, so I start to see the old man again, I shave him help her move him, feed him etc, I take him to his last hair cut and pull the wheel chair to the sun so he can get some air and warmth, and then I told him, even now that you know that this is the end of the road, and the end is near you can't apologize to me for being such a prick, or at least say to me take care of my daughter, he give me this **** you stare, no more, he died a week later.
I have peace of mind because at least for me I help the old bastard until the end and never expect a thank you, do what you think is correct for your wife, because you don't want to see her suffer, and if it's just the yard and that keep your wife happy, show her that you're better then your sister in law and be polite, again IMHO.