It is said that sisters and brothers, generations of relatives, I don't understand, why can't sisters do this?

I am two years older than my sister, and I have eaten and slept together since I was a child, and I naturally have no feelings to say. I used to think that the feelings between sisters were the best among sisters, because they were girls, they could go shopping together, make food together, and whisper together, but some recent events made me feel that it was not so.
Thinking about it, it seems that from the year my sister got married, the relationship between us has become a little delicate.
I was married two years before her, and at first I was afraid that she would feel snubbed, and I often asked her to eat, watch movies, go shopping, and every time I came home, I bought her a lot of food and use that she liked. At that time, her boyfriend who had been talking for three years had just blown, was in a bad mood, and did not have my enthusiasm for her, which I could understand, more considerate and caring for her.
Fortunately, the year before, she was also married, and her brother-in-law worked as a worker in a factory. Last year she gave birth to a girl, I was happy or something, rushed to the hospital to see her, but she looked lonely, said that I did not have a good life, did not give birth to a son. I immediately said that she was in what age, and now, everyone wants a daughter, how good a daughter is, kissing her mother, and being able to dress up beautifully.
She bowed her head and said in the end, but men all want a son in their hearts, and when you give birth to a dragon, your brother-in-law is so excited, and everyone says that I have a son!
At that time, I didn't take it seriously, thinking that she had just finished giving birth, was depressed, waited to be out of the hospital, busy every day, and had feelings for the little baby, naturally there was nothing. Unexpectedly, to this day, she is still saying that I am not blessed, and her son is better than her daughter after all.
This year's Mid-Autumn Festival, we made an appointment to go back to my parents together. According to the tradition, the Mid-Autumn Festival four kinds of gifts, that is to return to the mother's home for the Spring Festival with the same specifications, husband and I are university teachers, in-laws are also retirees, there is no pressure on the economy, buy things for my parents, I am also willing, so, in addition to a small box of mooncakes (afraid of eating sweet is not good, symbolically buy a small box), and bought dad's favorite wine, a box of tea, a box of bird's nest, are the best.
When we entered the door, my sister's family had arrived. While handing things to my mother, I told her how to stew bird's nest, and then asked my father to open the bottle of wine as soon as he ate, which is the most suitable for the elderly to drink. I was preparing to go to the kitchen with my mother to cook, my husband quietly pulled me a little, made a look at my sister's side, and whispered that you should say less.
I was a little surprised, as if I hadn't said anything I shouldn't have said. Looking at my sister again, it frightened me, her face was particularly ugly, although my son was already standing next to her and teasing his sister, she still frowned and was absent-minded. I had to be careful and go to the kitchen.
At this time, the son ran over, pointed to the table and said, Mom, I want to eat the big apple. That's when I noticed a bunch of things on the table: a box of mooncakes, a gift box of apples, a bottle of local very ordinary wine, and a large box of middle-aged and elderly nutritious cereals. The brother-in-law sitting next to me looked down at his mobile phone, and I suddenly realized what the problem was, and the holiday gift we brought to my parents far exceeded that of my sister's family!
I was originally a brainless, grinning person, but the relationship with my sister in the past two years has made me become cautious and constantly blame myself in my heart: how I didn't consider what my sister's family would bring!
That's it, I have to find a way to remedy it! Just when my son said he wanted to eat an apple, I quickly said, such a good apple, let's go and say to my grandmother, I will open it and eat it, okay?
Mom in the kitchen is estimated to also listen to the outside movements, immediately came out and said, eat it, thanks to your aunt and uncle to buy, grandma has not bought apples this year! My sister didn't look at her brother-in-law angrily and said that she hoped that he and I could buy something!?
During the meal, I did not dare to mention the drinking, everyone had their own concerns, the husband looked for a topic to talk with the brother-in-law, the brother-in-law was always silent more. Mom kept picking dishes for us, and I took the child in my sister's arms and let her eat well first. She quickly finished pulling the rice from the bowl, took the child and said, Sister, you eat it quickly. Having said that, he turned and went into the bedroom where we had both been in the back room.
After eating, when my mother and I were washing dishes in the kitchen, my mother quietly told me that you should go first after eating, your sister always felt that she did not have a good life without you, unhappy, I asked her to stay longer, willing to live at home for a few days. I hurriedly said good, my heart is also very uncomfortable, I thought that the sisters would love each other without saying anything until they became an old lady, but I didn't think about it, alas!
Mid-Autumn Festival to now, we have not seen each other again, in the middle I have called her several times, she is light, I am about to eat together, shopping, she is looking for reasons to push that it can not come, even I want to take my little niece to play at home, she said that wait until the child is older... But wait like this, not to mention the next generation of two cousins can not get along, even the two of us who grew up will become more and more distant.
I have heard people say that brothers and sisters have been relatives for generations, I still don't believe it, now I think it may make sense, just like a friend said, between sisters and brothers, sisters will always hurt their brothers, brothers either have to enjoy the care of their sisters, or like a man, they always have to protect their sisters, and after the sisters fall in love and get married, they will always compare each other in the open and in the dark, than the husband, than the children, than the income, than the wear...
Is this really the case? How can I be as intimate with my sister as I used to be, and when we are both old and toothless, we can laugh and talk about our childhood together on crutches?
(Picture from the Internet, invasion and deletion)