‘And immediately when they were come from the Synagogue they went into the house of Simon and Andrew, with James and John. Now Simon's wife's mother lay sick of a fever, and straightway they tell him of her. And he came and took her by the hand and raised her, and the fever left her, and she ministered to their needs.'
Leaving the Synagogue they naturally went to the home of two of the disciples, Simon Peter and Andrew, who presumably lived in Capernaum. James and John also went with them instead of going home. They were now permanent followers of Jesus. It would seem that this house was used as a base while they were in the area.
It is possible that they told Jesus about Simon's mother-in-law in order to explain the absence of a woman of the house to cope with their needs. This might suggest that Simon was a widower, or that his wife was away from home, and that at the time he relied on his mother-in-law, although 1 Corinthians 9:5 does suggest that Peter's wife was still alive. Or they may just have mentioned it casually. Whichever way it was Jesus sprang into action. Going into her room He took her hand and raised her from the bed and the fever left her immediately and she was able to see to their needs. It would appear that it was still the Sabbath. Jesus did not see Himself as bound by the Sabbath regulations of the Scribes and Pharisees, except in public when He did not wish to cause unnecessary offence.
That this made a remarkable impression on Simon Peter comes out in that he remembered the incident specifically, and it confirms the idea that Peter's reminiscences are behind Mark. Why else would such a ‘trivial' detail be remembered?