It depends on what kind of bridal shower you want to give. If she is registered, you can make this a family & friends shower. This would be when they get alot of household things. Things they really do need. You can have only so many pairs of black lace panties, so I would have a separate girls night out small bachelorette party for "naughty" gifts.
Bridal showers can be anything. You don't have to spend a lot of money to make it look expensive. Look on line for ideas, in magazines or ask others. I hope that the other bridesmaids are chipping in. If not, contact them immediately and ask them to co-hostess. Or find out what are they willing to help with (both monetarily and physically), see who will help with food, prizes, plates, etc. Don't put this all on yourself. Recruit, delegate and organize. Many heads and hands make light work.
Just be prepared for Bridezilla. I don't know your soon to be sister-in-law and I am sure she is a lovely person, but the younger the bride, the more drama the event. I speak from years and years of experience in planning weddings, being in weddings, going to weddings (I was in 4 in one year). Brides will tell you whatever you want to do is fine, just know they lie. Causally question her about her ideas. Just remember, you are the one paying for it, not her. If she is being unreasonable, speak up and tell her "this is my budget, I can't afford anymore, but I will make it wonderful." This is tricky situation because she is an in-law. I don't know what relationship you have with her, but I have found you need to be very careful in speaking with in-laws. Otherwise, you are the bad guy.
First, you need a list of names and addresses (or if everyone is computer literate, emails). If time is short, phone calls. You need to know how many people are possibly coming. Also, you have to pick a date to have it. The weekends are the norm, If on Sunday, make sure it is after church time. You then need a place to have it. If your home is not large enough, ask your church about using the hall or see if another relative will host it at her home. Outside parties are nice, but you have to rely on the weather to cooperate. Parks are nice if you can get a shelter.
Once you have the list of names (push the bride, her mother and your mother for names ASAP), the date and the place, SEND OUT THE INVITATIONS. You don't have much time since the wedding is July 4th. I personally feel email is the way to go, but older relatives and people don't use the internet and like invitations by snail mail. If you have access to computer and printer, by all means find some pretty paper and design you own invitations. Put at least 2 on a page and buy regular envelopes to mail them in. They have kits for invitations at Walmart, Staples, sam's, target, the dollar stores, etc, you have to decide what you want to do and how much to spend.
Decide what kind of shower. You can go with a theme, like her colors for the wedding or patriotic for the 4th wedding and your military brother or buy whatever looks good ON SALE!!! You will need plates, cups, silverware, napkins. The dollar stores are great for these.
Next, plan the food.
It does not have to be elaborate - chicken salad, cold cut platter, vegetable tray, chips, one big money saver- don't have pop, have iced tea, coffee or lemonade (from a mix). Don't forget a bag of ice. Or just cake and punch. The point is to have family and friends together to celebrate the marriage to be. Don't be afraid to ask family members to bring their "speciality". Most older family members would love to chip in.
Order a cake based on your colors and theme. The size depends on how many people are coming, usually 1/4 to 1/2 sheet cake is good. Ask the people at the bakery price and servings per cake.
Games - you can decide to do games or not. One good one is to have the bride leave and have people write down what she was wearing, down to the earrings. Whoever gets the most right wins. Any prizes, dollar store. There are lots of game ideas on the internet.
I know it is stressful to plan a shower. Just remember, It will be wonderful no matter what you do. If the bride starts giving you a hard time, remind her: The shower and the wedding are just frosting, Her and your brother's marriage is the cake (ie, which is more important, a perfect shower or a strong marriage.) Good Luck.