When you arrive in the studio it will be warmed to about 80 degrees, so you will want to plan ahead. Some clients like to wear to the studio what they'll be photographed in as a family and then once those images are taken, change into a more comfortable outfit. Something with layers if you get too hot you can remove or add as needed. If Mama is nursing, a top to feel comfortable in.
If you choose to have baby nakey for some of the family photos, just plan on baby having some accidents so a change of clothes is always recommended!
As far as what you and your family should wear for your photo, here are a few tips:
Avoid loud colors and patterns. Solid colored shirts work best to not distract from the baby. Make sure there are no logos or wording on the clothing as well.
Avoid a zipper or buttons where baby will be resting their head on Mama or Daddy's chest.
I like to recommend creams, whites, neutral colors, black. Not necessarily everyone wearing the exact same thing but so they compliment each other.
Undergarments for the Mama's: take care to choose undergarments that don't cause bulging. Avoid shirts that show bra straps.