Addressing STDs

How do you do it? They don't have to be as formal as the invites, right?

Can I just do John and Jane Smith and then just first and last name for single friends? Or should it be Mr. and Mrs. John and Jane Smith? (I'm not doing Mr. and Mrs. John Smith at all, for this or invites.)

I also have two sisters I am inviting who live together. should I do Jane and Mary Smith or Ms. Jane Smith and Ms. Mary Smith ??