I would check with both your officant and the wedding manager at the church (person you booked the wedding through) to find out what the policy is. Some houses of worship have strict policies on how much skin you can bare -- shoulders covered, capped sleeves, sleeves to the elbow, sleeves to the wrist. You don’t want to show up on your wedding day and be turned away...it could happen.
If you’d prefer to show more skin than the venue allows, talk to the bridal salon where you bought your dress about cover-up options for the ceremony -- options that can be easily removed for formal photos and the reception.
As for your mother-in-law: If it turns out the church doesn’t have as strict a policy as she does/wishes, it’s your wedding so wear what you want to wear and don't talk about your wedding dress in front of her. On the big day she'll be too busy greeting her guests that she probably won't notice or mention your neckline.