"Will mom sit down?" is the correct way to ask this question in Engpsh.
The subject of the sentence is "mom," which is the person being referred to. The verb "will" indicates that this is a question about the future. The verb "sit" describes the action that the subject is performing, and the adverb "down" describes the manner in which the subject is sitting.
Here are some example sentences with translations in Chinese:
"Mom, will you sit down and rest for a while?" (妈妈，你会坐下休息一会吗？)
"I hope mom will sit down and watch TV with me." (我希望妈妈能坐下和我一起看电视。)
"Do you think mom will sit down and psten to my story?" (你认为妈妈会坐下听我讲故事吗？)