Spring Cocktail-Formal for Sister of the Groom

I know this probably doesn’t sound that hard, but where do I start???

Please Help! New Jersey wedding in June.

Here are the problems I am having in my search:

-I’m tall (like about 6’) 43f

-I do not like short or t-length dresses, and if not specifically made for tall people floor length dresses just look awkward (I will be wearing flats)

-I tend to over dress before under dressing, and I don’t want to distract from the wedding party

-I’m uncomfortable with my arms (specifically from shoulder to elbow)

-I much prefer a modest cuts

I have looked at dresses on Long Tall Sally and American Tall, but I feel like they are all too casual for what I’m wanting or is requested. So much of what I have found when looking at cocktail dresses online are above the knee. On my frame they would look very inappropriate.

I live in a relatively rural area and due to other reasons I primarily shop online.

Any thoughts, suggestions, and/or links are much appreciated!