What should a sick person, who is incapable of fasting, and who didn`t make up for missed fasting days, do?

One who has missed fasting days is obliged to make up for them, but if he/she wasn`t able to because of an incurable disease , or old age, then he/she has to pay a ransom which is feeding a needy person for each of the missed fasting days. And Allah Knows Best.

Is it permissible to make up missed prayers at the times in which praying is disliked?

All perfect praise be to Allah,The Lord of The Worlds                                                                                                                                                                        It is permissible to make up missed prayers at any time even that in which praying is disliked. Absolute supererogatory prayer is forbidden to be performed at the times in which praying is disliked, and this also applies to Ihram and Istikhara prayers. However, there is no harm in offering prayers in Mekah during those times. And Allah Knows Best.

Is it permissible for a person to sever ties with his sister because her husband had quit prayer and done him harm?

A sister is a blood-relative and maintaining blood-relations is obligatory in Islam. As regards her husband, he should be invited to the way of Allah with wisdom and beautiful preaching.

Allah The Almighty Made marrying up to four women lawful, but human beings prohibit that with their own land laws. What is the position of Sharia on this?