As someone who used to suffer the painful, patronizing unwanted attentions of an interfering, all-knowing mother in law, I am very well versed in "dealing." As a mother in law myself, I am well-armed with what NOT to do, having learned the hard way from an expert. So here are a few tips that will hopefully help to make what can be a difficult relationship into an amiable, mutually respectful one - the ABC of successfully dealing with a mother in law.
ACCEPT that there may be some feelings of jealousy on her part, because somebody else is now more significant in the life of the beloved son or daughter.
ASK her advice on little matters, be it knitting, planting carrots, cooking or choosing a gift for the mutually loved one. You might not need this advice, but just asking makes the mother in law feel wanted and valued.
BE open to suggestions, gifts and ideas from the lady, with regard to your partner. She has, after all, known them longer than you have. She loves them as much as you do, too.
BRING her something when you visit, some little token. Buy her lunch once in a while, or even make a regular date for coffee and chat; that means you will get to know each other better on neutral territory.
CURB you resentment when she "interferes." Instead, tell her how glad you are to have a second, more experienced opinion, even though you feel like slapping her.
CALL on her in a crisis. Yes, this is one of the best ways to establish and cement a friendship. You will find yourselves working together to deal with the trauma and this brings you closer to real friendship than you could ever have imagined. I speak from experience, so this is true.
DARE to disagree sometimes, but in an amiable, yet assertive, information-seeking way. Turn it into a discussion, not an argument. She will respect that you trust her enough to share your conflicting thoughts and ideas.
DO NOT criticise your spouse or other members of the mother's in law family. It is fine for her to have a moan and a gripe, but never get embroiled with this, just nod sympathetically in the right places.
Every mother in law is first, a mother. You would hate it if your mom was hurt or put down in any way, so think how you would want her to be treated by her sons and daughters in law. Apply that to your own mother in law. Believe me, in time, you will make yourself a really good friend. At any rate, that was my experience, despite our radically different personalities and approaches to life. I still miss her, ten years on, but I have all the flowers and plants she put in for me when I got my first garden. I see her in them and I am glad to have had her in my life. Dorothy, my lovely clematis.