What is the ruling of Sharia on marrying the maternal niece of one`s wife?
All perfect praise be to Allah the Lord of The Worlds. May His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
One may not combine in marriage a woman and her paternal aunt, or a woman and her maternal aunt, as stated the Prophet (PBUH). And Allah The Almighty Knows Best.
Is permissible to buy/sell with non-Muslim?
Yes buying/selling for a Muslim with non-Muslim is permissible as regards what is lawful.
What is the ruling on aborting a fetus affected by thalassemia?
Praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
If this is recommended by a trustworthy doctor who is god-fearing and knowledgeable, then it is permissible to have an abortion because some types of thalassemia are dangerous while some aren`t. And Allah The Almighty Knows Best.
Is it permissible to fish on Saturday?
All Perfect Praise be to Allah, The Lord of The Worlds, and may His Peace and Blessings be upon our Prophet Muhammad and upon all of his family and companions.
It is permissible for Muslims to fish on any day, but the sons of Israel are the ones forbidden to fish on Saturday. Moreover, there is no evidence in Islamic Sharia prohibiting Muslims from fishing on that day. And Allah Knows Best.