It can even be said that if we seperate the cost of contracting Mitsuri, Liu Mengmeng's travel to Demon Slayer world is almost free of system points.
Coming back to the topic, the reason Liu Mengmeng asked the sibling if they want to go with her is the relationship she built with Nezuko during her stay in Demon Slayer Corp and their character and deeds.
Tanjiro can be said to be one of the best protagonist of anime series in her previous life, at least she thinks so. He is loyal, kind and understanding but only within limit of his tolerance.
He has indomitable will, strong combat talent, extraordinary tactical intelligence and excellent combat instincts. He is extremely determined and hardworking person who is unafraid of difficulties.
Liu Mengmeng felt that Tanjiro was an excellent person who is actually restricted by the limit of the world. Given chance, he will definitely shine brightly.
Nezuko is also an excellent individual. She is the first person to supress her demon instinct by sheer willpower. Nezuko is great beauty and she is kind and caring. She also posses extraordinary combat talent.
Tanjiro and Nezuko, who were sitting opposite to Liu Mengmeng didn't agree to offer immediately but they didn't refuse either.
Nezuko could feel that others always had a slight guard against her because of identity as a demon. The only one who never had any guard against her are only two people. One is her brother and other is Liu Mengmeng. So she tends to agree but left the decision to her brother.
Tanjiro, on the other hand was recalling his time in Demon Slayer Corp. He found that Liu Mengmeng had indeed taken good care of him and Nezuko. And with his extrasensory perception, he never sensed any bad feelings from Liu Mengmeng towards them.
Tanjiro and Nezuko, signaled each other with eyes and finally Tanjiro decided to answer positively. As for what reason, Liu Mengmeng wants them to follow her, Tanjiro didn't think much about it.
He thinks that whatever Liu Mengmeng wants them to do, they must be able to do it otherwise she would never invite them.
"It's our pleasure to follow you, Lady Mengmeng." Tanjiro and Nezuko said in a formal tone.
"You don't need to be so formal. Just call me Mengmeng." Liu Mengmeng said happily. She purchased two contract cards and signed contract with both of them.
After doing all this, Liu Mengmeng took out three energy bodies and sent them to Mitsuri's, Tanjiro's and Nezuko's bodies. Apart from the one made for Mitsuri two years ago, the other two Pokemon martial souls were made by her few days ago for the two siblings.
"Now listen carefully." Liu Mengmeng said in a serious tone. And hearing the change in her tone, other three people in the room sat upright and focused their attention.
"I came from another world and you will follow me to that world. After a while, you will go to a unknown village and your bodies will revert to six year old. Don't panic about it and live in the village patiently for few days." Liu Mengmeng paused and looked at their expressions. All three of them have a doubtful face.
"I know it sounds weird but just listen to me. In few days the children of six year old will gather for martial soul awakening ceremony. Remember you have to definitely participate in it. Don't miss it at any cost."
Seeing them nod, she continued, "When you will awaken your martial soul, only show one and hide the other. Remember to show the animal type martial soul. Your soul power will definitely be very high so the person who will awaken your martial soul will ask you if you want to join the Spirit Hall. Just agree to him and come to Martial Soul City, I will meet you in the city."
After making sure that three of them have properly remembered her instructions without any mistake, Liu Mengmeng was relieved.
"Take this and write whatever you want to say to the rest of the people of Demon Slayer Corp." Liu Mengmeng gave three of them a piece of paper and let them write their goodbyes.
"There are more paper here, so write freely." Liu Mengmeng took out sheet of papers and motioned them to it freely.
All three of them wrote many letters and folded them properly and wrote receipents name on the blank side.
"Are you done?" Liu Mengmeng asked after seeing them stop.
"Yes, all done." All three of them replied.
"Then be ready for your new life" Liu Mengmeng said and took out the card from Space Ball whose countdown is ending. Soon the card shone brightly and four people disappeared from the room leaving a bunch of letters and a copy of Normal type Breathing method on the table.