One or the other are sufficient. The one exception may be if you are particularly close to the mother and child, such as an Aunt or the Godparent.
Motherszone gives perfect baby shower gift ideas for both mom to be and her baby such as Personalized clothing hanger, Modern Diaper Bag and many more.
A shower for a baby boy and a shower for a baby girl are both termed a "baby shower". There are different party themes that you can do for a girl baby shower. For example, you could throw a princess baby shower, a pink baby shower, a cupcake baby shower, a flower baby shower, a ladybug baby shower, and many other themes. See related link below for more inspiration.
If you are in the US then this is a great site http://www.simplyuniquebabygifts.com they provide lots of baby shower gifts for both sexes and unisex, they also offer free delivery to many states. You could also try the online department stores or Amazon.
Babies R Us is a store dedicated to baby items. There is also Buy Buy Baby which is affiliated with Bed Bath and Beyond. Both stores offer many items for babies and young children.
Both parent's names should be included on a baby shower invitation if the party is a couple's baby shower (sometimes called a coed baby shower). Otherwise, only include the mom's name. For examples of couple's baby shower invites, see related link below.
A couple's shower involves guests of both genders (a regular baby shower ususally involves the mother-to-be and female guests.) A twin baby shower celebrates the birth of twins.
A baby shower is typically gifts for the baby and thus, whatever you choose to give your sister, will most likely end up in the hands of the baby. Whatever you choose it doesn't need to be too, fancy; it should be just enough however to please both your sister and hopefully the baby. My sister was invited to a baby shower and asked me for gift ideas, after being asked my opinion I was quick to point her in the direction of this website: http://www.gifts.com/ideas/baby Remember, just enough to please your sister and the baby.
A gift or a card, or both.
Duck themed baby shower is great for a baby shower. Duck is a gender-neutral theme that works for both boys and girls. http://www.newbornbib.com/top-menu/articles/throw-a-baby-duck-themed-baby-shower/
A gift or a card, or both.
Both sisters should put their heads together and work as a team and have the baby shower for their sister. Both sisters should get equal thanks for their efforts.
Baby shower ideas can be found all over the place but a good site is simplybabystuff.com another good site is babyshower101.com Both of these sites have good ideas for planning your baby shower.