MIL doesn't dress poorly. Her wardrobe is professional and serviceable, but it also leans toward monotone and monotony. Here's my attempt to break her out of her fashion rut and launch her from good to better.*
This suit look is both perfect for an interview, and if you're in a conservative position, a great everyday look. Both if you want to (and can) have a little more fun, you have plenty of options with this base. First of all, you can add color. I'll say it over and over. Color adds interest and makes you look more alive. If you are afraid of color, stick with the jewel tones (emerald, sapphire, ruby and amethyst). Here I switched the pearls for a scarf, the watch for a ring and black pumps for red. If nothing else, buy shoes in a color. They always make it look like you thought about your outfit even if you grabbed your purple mary janes at random to wear with your brown suit. A second, more conservative, option is to add small details. A good belt makes outfits look finished. If you frequently tuck in your shirt, buy a belt. The only other thing I did here was trade very basic black pumps for black shoes with more punch.
Basics are great, but if you layer them with other basics...well, you look boring. Each piece in the look below is excellent but not together. The key is to add texture. Swap the cardigan for a chunkier knit or the button down for a ruffled blouse and voila, instant visual interest.
MIL's usual treatment of brown is to wear more brown. This can be okay if you spice it up with your accessories. Instead of a dainty necklace, choose bigger jewelry -- a layered necklace, brooch or cuff. When colors are quiet, remember the importance of shape. If you're wearing a loose top, a belt worn at the waist or hips will create a more defined silhouette.
Those same lessons can be applied to the brown-out below. Trade plain tights for textured ones and upgrade the tame shoes to a very professional croc pump. If you constantly wear blazers, pin brooches on all of them so you don't even need to think about statement jewelry. Again, you can pump up an outfit with color. (I'm particularly fond of this scarf as you know without thinking that you can wear it with brown and blue.) Finally, you can add both texture and color with a print. A print blouse will be the workhorse of your wardrobe.
*I asked my mother-in-law what she'd wear with whatever I was styling and showed her the "good" before styling the "better". She was thrilled with the results.