I know this thread is old but I've enjoyed it so so much and wanted to put my two penneth in (and hopefully kick start it to get some more funny idiosyncrasies).
DM leaves price tags on every single present she buys. Even if it has an enormous red 'SALE' tag on it. And she says the same thing every time - "whoops did I leave the tag on? Silly me!". Think she does it to secretly benchmark how much we need to spend on presents lol.
And the faffing - oh, the faffing! Going anywhere needs a 1 hour lead time from saying "shall we set off?" To when we actually leave the house. Much turning off of appliances, checking opening times online (even if were going to the 24 hour supermarket she must check this hasn't changed back to a normal one), discussing the best route to avoid traffic, putting on coats and jackets, taking off coats and jackets because it "looks quite sunny actually" - and then she says "come on, we'll be late!"
She also has to stop constantly on the motorway. She must have the bladder of a gnat (and she's only 55!). We went from Manchester to Gretna once (about 2 hour 15 mins journey) and stopped 4 times. Kids aren't that bad!
DMil will wait until you've finished eating, literally the second you put your knife and fork down she whips your plate away and washes it. Even if she's still eating herself. Cooking a simple dinner (beans, eggs, sausages for example) can take an hour. God knows what she does! She doesn't really cook much so never had a roast at hers or anything, I imagine she'd need around 12 hours to make it though!
DStepMum does the food counting thing too! Guaranteed Sunday dinner means the following (NB all the following come in miniature form, I've never seen such tiny meat & veg in my life, I genuinely don't know where she buys such small food) - either leg or breast (one each only), two Yorkshires, 2 roast potatoes, 2 florets of broccoli and a teaspoon full of gravy. It's really tasty though I wish there was more. And it's always always followed by peaches and ice cream. I'm not sure she's aware that other desserts exist.
I love them all dearly though (and the food all round is yum so I won't complain ever out loud) and wouldn't change them for the world, I agree all these are very endearing qualities rather than annoying!