Black tie dress code at wedding

Risa Marie • Wife. Farmer. Momma to Elsie Jo & Sadie Mae. Cat Mom of 2.
I've never been to a black tie event and this September DH and I and our baby girl who will be around 2 months old at that time are going to my DH's Brother's wedding in Georgia. I have only met his brother once and that was at our own wedding, i've never met his fiancée my soon to be sister in law and i'm afraid of upsetting or offending her if we aren't dressed appropriately for this black tie wedding. No one i am close to could really help me with this so i was hoping someone on this app would have some experience. I have no clue what is appropriate for a 2 month old girl to wear to a black tie event and also where to find a black tie appropriate dress for myself that will be easy to breastfeed/ pump in?? My friend told me no one is going to tell a 2 month old they are underdressed but i'm just worried because i've never met the bride to be. I plan on using bottles to feed the baby at the wedding if i have enough pumped but i figured at some point in the night i'd probably have to pump or something. TIA