You want to do a great job for your bro, but you've got to keep some perspective. No one's coming for the food. The typical wedding menu means choosing two out of the three: fish, poultry, and mean; plus making sure there's a vegetarian option, etc., etc., etc. Admit it. Kids' birthday parties are more interesting.
You've already tried six with good reputations and all have presented food with egregious technical flaws? You may have hit them on bad days, or you may have very high standards. Speaking as an old (ex) caterer, after four or five I'm wondering about you.
What are their favorite restaurants? Some provide catering or may be able to point you to a relative... I mean someone they know who's outstanding. One of the good thing about choosing a caterer based on the wedding couple's choice, is that if something goes wrong it's their fault.
If the wedding is not too large, you don't need an enormous amount of wedding-specific technical support from your caterer, and it's possible in your area, you may want to consider an ethnic or specialty caterer (Chinese, or Soul Food, e.g.), and not be afraid of ethnic places when you go through the list of potentially appropriate restaurants. In any case something they both like.
The idea here is that the food will be exotic enough that if there are complaints you can chalk them up to what a bunch of unsophisticated, intolerant, ignorant hicks HER side of the family is.
Who doesn't like Thai?
Have you ever been to a Chinese wedding banquet? Oh. My. God. Not cheap, though.
In all seriousness, it's the specialty caterers who are all about food -- as opposed to service, presentation, co-ordinating your rentals, and so on that the big guys do oh so well along with doing a decent job on more mundande menus.
Speaking of the big guys, they probably won't be able to meet your highest foodie expectations (have you ever cooked for 200?) in terms of providing a "fine dining experience" (it's usually a mistake to ask for one) , but you should be able to work out a seasonally appropriate, not too hackneyed menu which will at least please you conceptually. The simpler you keep it, with the fewest number of cream sauces and passed hot apps -- the better it will all work out.
Try and get at least one dish on the menu just for your lovely new sister in law and one for her PITA vegan friends. She'll appreciate the thought.
For God's sake serve decent champagne. That will cover a multitude of sins. Korbel Brut at a minimum. No "black-bottle" Freixnet; no Asti-Spumante; no Prosecco; no $5 Trader Joe's specials; nothing too sweet or too cheap.