I think above all focus on sincerity. If you do things with goals and expectations (if I do x, y will result) she will feel stressed and pressured to produce y, even if she doesn't want to. However, if you act sincerely polite to her sister, listen to her, and try to make the relationship between you and her smooth for your girlfriend's sake, your girlfriend will definitely pick up on it an appreciate it.
For me, the less explicitly said, the better. If you told me "I took the three of us out to lunch so that you'd know I was trying to get along with your sister," I'd take that as an expectation that I had to in turn give something to you. But if you just do nice things for her to show her you think about her and care about her without trying to prompt a verbal reaction from her, she might appreciate the gifts and attention more because they don't have some kind of hidden expectation attached.
Also sometimes someone telling me how I feel or think, even if it's to show me that they understand me, can be tricky. If they're wrong, it seems like they have some kind of image of me that I don't relate to at all and they don't understand me or how I work. If they're right, it makes me feel exposed and vulnerable, but sometimes deeply touched. So be careful about that.
Maybe this is too cautious, but I'd frame my guesses in the form of a question and then ask her to confirm it, like: "Is it because of x?" "Do you feel y about it?" If you're right, it gives her the opportunity to elaborate, but if you're wrong LISTEN to her explanation/clarification why (even if you disagree). Don't ever just tell her how she feels or why she feels it, even if that's how you're framing your guess, because she's the only one who can really know that and if you don't listen to her she'll think you don't care about her or knowing the "real" her that she doesn't show everyone.
Surprise gifts with no strings attached are very nice, so is doing nice things for her and her family, since oftentimes family (moreso if they're close) can be an extension of the inner self for the infj. One of her big concerns is how you're going to mesh with that, so the more you show that you're aware of that/have care about that the better in my opinion.