Too close to mommy

We have 1 child who is turning 4 soon. She has problem adjusting to friends at school. Her social skill is very poor. One of the reason is she never play with other children before she started school. Her surrounding is only with adults. So we are going to bring her to see therapist next week. My mil said that she is too attach to me (Im SAHM and the main caregiver - fully breastfeed/direct latch until 3.5 yo). We usually bring her to see my mil once a week. Now her comment is 'you need to let go' meaning she wants me to put my kid at her place 2-3 hours a day. Ok my emotion is I dont want to let her taking care of my kid because I dont trust her. And Im sahm so i can take care of her myself. My question: does this attachment to mother really effect the kid's social skill?