Your SiL probably sees you as having all the time in the world, being retired, so it's perfectly easy for you to give him help when he needs it. But as he's working, he doesn't feel obliged to reciprocate.
This is wrong, and very unfair because what he's not considering is that these things are very much harder to do as we get older, so it's not just about the amount of time involved, it's the effort as well. What he would take a couple of hours to do might well take you all day.
Trouble is, our kids, including our kids-in-law, just see us as they always have - the parents who've always done things, always coped, always been there for them. I don't think it really registers fully with them when we start to slow down and have health issues which make life difficult for us. And consequently it takes time for them to recognise that actually they need to start 'being there' for us.