The sequence of events here is complicated. The Greek order for the first part of the verse is as follows:
(1) I am afraid—
(2) (the demon) doesn’t hurt her,
(3) (but the demon) kills anyone who wants to get near her;
(4) I am my father’s only child—
(5) (lest) I die and bring the lives of … to the grave.

Grammatically, clauses 2-4 are interruptions in the flow of the narrative from 1 to 5: “I am afraid lest….” New Revised Standard Version rearranges the order to 2-3-4-1-5. In Good News Translation the order is 2-3-1-4-5. It could conceivably be argued that the order in the Greek text is intentional on the author’s part, showing Tobias frantically babbling at the prospect of marrying this girl. Translators should carefully study these three ways of arranging the order of the events and decide which will be the most appropriate in the their language. If the Greek order can be maintained, the emotional impact will probably be greater.

Who desires to approach her: This is probably a euphemism for “have sex with her.” Good News Translation “who tries to get near her” refers to sleeping together on the wedding night. Contemporary English Version “who tries to marry her” is not correct—the couple were already married when they entered the bridal chamber.

Bring my father’s and my mother’s life down to their grave, grieving for me: may be rendered “My parents would be so sad that they would die.”

No other son: Apparently they have no daughter either. So this could also be expressed as “no other child/offspring.”

Quoted with permission from Bullard, Roger A. and Hatton, Howard A. A Handbook on Tobit. (UBS Helps for Translators). New York: UBS, 2001. For this and other handbooks for translators see here.