Have you and your fiancee considered getting pre-marital counseling? I'm not sure it will take, but it will give you some different eyes on your proposed future, and perhaps some tools to use as you move forward.

Don't discount what your family is telling you, they know you. They aren't kidding when they say marriage is hard. They are telling you something important.

There are two marriages in my mind; the one you think marriage is, before you do it, and the reality after. (joke)

Getting help before will help the two of you open any unopened doors, make sure you are on the same wavelength about the important 'to you' issues.
Do you agree who will manage the money, children, illness, household mechanics, chores? Saying you will work it out as you go is not an acceptable answer before marriage, it is the best answer after. You need to know a lot about what each other expects before you enter this.
I take marriage very seriously, it is a huge commitment. Be sure you both understand what it is and what it means to yourself and that you really understand what it means to your partner.

Being clear and open is the key to unlock this door to happiness.

Best of luck to you both! Young and in love is lovely!