That's a good question.

Of course, this is just my opinion. Hopefully, some others will give their advise as well.
I think it would look best if you can find something tea length. If you can't find anything tea length (or have a long dress shortened to tl), then a shorter dress length would be ok. If you do a shorter dress, then try to get one that is on the "longer side"...like you mentioned. being at (or just below) the knees.

Do you know what color(s) the mob is wearing? The type of fabric? What color(s) are you looking at for your dress?

I had a daughter married this past April. Another daughter will marry this coming July.

I am a MOG (wedding in September), too.

I don't think it makes a bit of difference if your dress is cocktail length even though the MOB's is tea length, no more than if the sleeves were different lengths or something. They really are the same idea -- a party dress that isn't an evening gown. It's just a question of which length you like better.

You can always ask your future daughter-in-law if she has any objection. If she does, I would think it's silly, but I'd do what she asks -- it's her wedding, and a dress is nowhere near as important as your relationship with her.

I was stressing out over the very long odds of finding a green evening gown, as my FDIL had requested. But when I told her I was having a tough time finding something long AND green AND petite and asked her if she'd mind if I expanded my search to include other colors that blend well, like beige or navy, she said sure. So you may be worrying for nothing.

Congratulations! And to you, too, laurie 2008.

I agree with Gellchom. As long as your dress is not too short and it isn't full length, you should be fine.

Yes, definitely check with the bride. Also consider how that will look in the photos. If one person really stands out from the bride and groom or others in the shot, a good photographer will put that person way in the back.

We had one MOB who wore a red dress with a giant flower pattern on it. The rest of the bridal party was in pastels, so the eye goes directly to the giant red flowers. Not pretty.

What you are trying to match is the level of formality of the MOB's dress, not her exact dress type. Both tea length and cocktail length are pretty much the same level of formality, sort of a "semi-formal" look.

If one of you were wearing a floor-length dress and the other a knee-length outfit or a mini skirt, then maybe there'd be a issue. But the few inches between knee-length and mid-calf? Shouldn't be a problem.

To say nothing of the fact that tea-length is very flattering on some women and not so very flattering on others. Same with cocktail length.