But helping out around the house- that stuff is cyclical and something always needs to be done, no matter how on-top you are. I don't think you need to clean her bathroom or anything but if you have dinner at her house you can hop up and clear the table/do the dishes before she has a chance to. In terms of the "love languages," ISFJs tend to speak in "gifts of service." It's how they show their love and they consider it an act of love from others. But you're right that it's important not to do it in such a way that it weakens her, in her own eyes.
Again I want to say how sweet it is of you to be thinking in these terms. And I really get the impression that you're more interested in being a comfort and a relief to her, rather than getting positive strokes for yourself for being so helpful. She's very likely to see that.
Oh, one more thing-- it feels great to be praised for things like how I parent or run a household, but it also feels great to be praised for being intelligent or clever or funny, things people don't always expect from Martha Stewart-types. (Not that I am in ANY WAY a Martha type, but I think sometimes people IRL think I begin and end with my parenting and homemaking, and I wish they knew there was more to me than that. Which is probably why I spend so much time here.)