First off you will never be able to know what every individual will want/eat. Since this is for family, and not a contracted catered event, I would make extra to ensure that there is plenty for all. You can always take any leftover amounts home and enjoy your Q for a few more days!!!
Our formula (brisket or pork) for this type of event is to use a 50% weight loss in cooking and add a 20% buffer on top of that. For 53 folks @ 6 oz servings each you need 318 oz cooked (just under 20lbs). Double that (40 lbs) and add 20% (8 lbs) for a total of 48 lbs raw weight. This will yield approx 64, 6 oz servings (cooked).
As for the mix of beef/pork this is again almost impossible to hit dead on. In our area beef is KING. With that being said we do events where we cook a 50/05 mix and sell out of pork early
. It depends on your area and peoples personal tastes. Also, even though we may sell out of one of the meats first we have NEVER had anyone turn away saying they will not "settle" for the one we still have (our Q is THAT GOOD...lol). I would suggest a 50/50 mix and just let the chips fall where they may. Therefore 24 lbs (raw) brisket and 24 lbs (raw) pork. Using the product we have in our area that would equate to 2 briskets and 3 butts (your mileage may vary concerning raw weight of butts/briskets).
It sounds like you have the sides covered and sounds like a nice spread you have planned. Let us know how it turns out!!