Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 8 · Marriage
विवाहः
विवाहश्चैकजन्यानां षण्मासाभ्यंतरे यदि ।असंशयं त्रिभिर्वर्षैस्तत्रैका विधवा भवेत् ॥ ५१६ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 56, verse 516.
Say two weddings of children of the same mother fall within six months. Then within three years one of the women may face the loss of her husband.
Simple meaning: Two weddings of siblings within six months may bring one woman the loss of her husband within three years.
प्रत्युद्वाहो नैव कार्यो नैकस्मै दुहितृद्वयम् ।
Do not do an exchange wedding. Do not give two daughters to one man.
नैवं कदाचिदुद्वाहो नैकदा मुंडनद्वयम् ।
Some weddings and rites must not be done twice together. Birth at certain times may risk a parent's life.
आत्मानं संध्ययोर्हंति नास्ति गंडे विपर्ययः ।
A twilight birth may risk the child's own life. A Moola birth may risk the father-in-law's life.