I'm aware of those portions of Scripture. I'm also aware of what it takes to remain married for a lifetime. Besides, that Scripture passage you quoted does not mean we are to cut ourselves off from those who are loathsome to us, only a warning that our faith will make us loathsome to some, even our families. (See below.)
You do marry the whole family. You can fly thousands of miles away, but your in-laws are still your in-laws, your wife's parents. I do not know a married couple who does not say the same, even those who have parents so pathologic that no safe contact is possible.
Families with the problems you describe are not so rare as you think. MIL is probably not going to "get help." Chances are, she is going to be her irrasible self for as long as she lives. You're just going to have to cope with that. You can put some distance in, but are you saying you and your wife are not going to go to family weddings and funerals, are not going to have anything to do with her relatives, because you're too thin-skinned to deal with her mother's fully-predictable tantrums? How will you "weep with those who weep and rejoice with those who rejoice" if you do that? What if God put you in a room with her for eternity....could you love her? If you cannot love your enemies, how can you become a saint?
*The saints are what they are, not because their sanctity makes them admirable to others,
but because the gift of sainthood makes it possible for them to admire everybody else.
It gives them a clarity of compassion that can find good in the most terrible criminals.
It delivers them from the burden of judging others, condemning other men.
It teaches them to bring the good out of others by compassion, mercy, and pardon.
We become saints not by the conviction that we are better than sinners,
but by the realization that we are one of them,
and that all together we need the mercy of God!
Thomas Merton, New Seeds of Contemplation*
You don't get to be a saint by saying, Thank you, Lord, that I am not like other men, not like this mother-in-law of mine, greedy, loud, quarrelsome, self-centered....
You get there by saying, "Lord, if you can put up with me, I can surely be patient with her, for you love her and thirst for her as you do me, and your patience with me has been without measure..." If you're aiming for canonization, that is the way the saints think.
"*But to you who hear I say, love your enemies, do good to those who hate you, bless those who curse you, pray for those who mistreat you. To the person who strikes you on one cheek, offer the other one as well, and from the person who takes your cloak, do not withhold even your tunic. Give to everyone who asks of you, and from the one who takes what is yours do not demand it back.
Do to others as you would have them do to you. For if you love those who love you, what credit is that to you? Even sinners love those who love them. And if you do good to those who do good to you, what credit is that to you? Even sinners do the same. If you lend money to those from whom you expect repayment, what credit (is) that to you? Even sinners lend to sinners, and get back the same amount. But rather, love your enemies and do good to them, and lend expecting nothing back; then your reward will be great and you will be children of the Most High, for he himself is kind to the ungrateful and the wicked. Be merciful, just as (also) your Father is merciful.
Stop judging and you will not be judged. Stop condemning and you will not be condemned. Forgive and you will be forgiven. Give and gifts will be given to you; a good measure, packed together, shaken down, and overflowing, will be poured into your lap. For the measure with which you measure will in return be measured out to you*." Luke 6:27-38
"Bless those who persecute (you), bless and do not curse them. Rejoice with those who rejoice, weep with those who weep. Have the same regard for one another; do not be haughty but associate with the lowly; do not be wise in your own estimation. Do not repay anyone evil for evil; be concerned for what is noble in the sight of all. *If possible, on your part, live at peace with all.** Beloved, do not look for revenge but leave room for the wrath; for it is written, "Vengeance is mine, I will repay, says the Lord." Rather, "if your enemy is hungry, feed him; if he is thirsty, give him something to drink; for by so doing you will heap burning coals upon his head. Do not be conquered by evil but conquer evil with good*." Romans 12:14
Of course you can put some space between yourself and your enemies, those who want to do you harm. Christianity does not demand masochism. Having said that, those of us who have been around the barn a few times know what you're getting yourself into. Do not imagine that you can walk away and just act like MIL does not exist. It does not work that way. You asked what we know: We know this. Accept it.