Big brother is selfish and heartless. Sister-in-law is very calculating. I'm my sister-in-law They are cruel and selfish to me, but I am still kind to them (how stupid I am).
In fact, you can also think in another way. They are elders and naturally know more than you. You can ask them what they need when you buy something. Maybe they are tactful and picky because they don't want you to spend all your money on things that are not practical. You are kind to them from the heart, and I believe they will not be so heartless.