Tell your sister to snap photos constantly, the whole time you are all together-I think the best kind of "family portraits" are the candid kind-when you are naturally interacting as a family-laughing together, dad scruffing his son's hair, the baby reaching for her mom's face with the mom making a silly face....or playing with leaves together, etc...the kinds that really SPEAK of the relationship and bond you share as a family.
But, if you are into posing, then have you and your husband sitting, leaning sort of on an angle in towards each other (DON"T have your husband sitting behind you-that's such a cheesy-husband-as-head of the home look...it's really outdated), with the baby perhaps sitting on one of your laps, with your son standing between the other parent's legs...by having you on each side of the frame, it is subconsciously projecting this image of having you two framing and protecting your children-holding them between both your hearts...
Also, don't worry so much about being wildly matching...no need for that. But, if you already bought them, it's fine.
I'm a professional photographer