Good for you, you mean ole daddy!
Now granted, some girls ARE indeed mature enough to get married and run a home at 16, but you know her better than anyone here.
If you want, set some guidelines for her to PROVE herself to you as mature enough to keep a home. Seriously offer her the responsibilities of running your home for a month or two - just remember she is learning and will not be perfect. Her willingness to learn will be your indicator, not the actual "perfection" of the job completed.
Plus there's the matter of her husband-to-be:
Where will he provide her a home?
How will they pay for it?
Will she finish high school? And will she wait until after high school to have a baby?
Have them sit down with you and explain their plan for the next five years. If they have thought it out, then really listen to them. If they have not, then tell them to think about it and present one to you.
She needs to know that you love her and are really watching out for her. It is your job as her daddy to protect her and teach her until she IS old enough and mature enough to be on her own through marriage or independently.
Definitely will be praying here for you.....