Is it ok to give a loan to your family without a lawyer?

If you are talking about any significant amounts here, do yourself a favor and get a lawyer. Generally, family loans end up not being loans but gifts; do not kid yourself in believing otherwise. So consult with an attorney to see if this "loan" really makes sense. Protect yourselt by getting an objective third party opinion about the advisability of entering into this transaction.
From a personal perspective, you need to ask yourself what would happen if you just said no to this loan. Or if you are really brave ask your fiance directly-you may be shocked at the answer.

Hope this helps.

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Based on your "repayment" terms alone, I think that you should consult an attorney if this is more than a few thousand dollars in monies. Generally speaking you would want more detailed and robust repayment options with acceleration clauses and other protections, if you require repayment or need to enforce against non-payment. Also, there is a gift tax issue that you will have to deal with if you do not charge interest.

I hope this helps.

Disclaimer: This answer is for informational purposes only and does not constitute general or specific legal advice, nor create an attorney client relationship.