Perhaps Peter's mother in-law had some contention.

It could have been over Peter leaving his nets to follow Jesus.

Mark 1:16 Now as he walked by the sea of Galilee, he saw Simon and Andrew his brother casting a net into the sea: for they were fishers.

 17 And Jesus said unto them, Come ye after me, and I will make you to become fishers of men.

 18 And straightway they forsook their nets, and followed him.

She might have been concerned for her daughter's welfare.

If so, then Jesus had a great way of turning it all around:

Mark 1:29 And forthwith, when they were come out of the synagogue, they entered into the house of Simon and Andrew, with James and John.

 30 But Simon’s wife’s mother lay sick of a fever, and anon they tell him of her.

 31 And he came and took her by the hand, and lifted her up; and immediately the fever left her, and she ministered unto them.

She would have known, then, that Jesus would take care of their needs.

Matthew 6:33 But seek ye first the kingdom of God, and his righteousness; and all these things shall be added unto you.