Hi B. -
I'm sorry to read what you're going through with your in-laws. There seem to be many, many moms on this site who are experiencing significant troubles with their in-laws, and it's very unfortunate.
Regarding your question on the invites...here's a very practical way to practice FREE etiquette (especially if you think they won't come anyway). Instead of wasting your money on postage, simply collect their e-mail addresses and e-vite them - here's a great, FREE site where you can do that: www.evites.com. If your in-laws aren't too keen with computers, shell out the postage, but in either case - whether e-viting or snail mailing - make sure you list where you're registered. If they don't want to spend money shipping out a physical gift, they could always order you a gift card or just send straight cash.
Now, about the favoritism issue...you and your husband have certainly been dealt a bad hand, but what can you really do about it? People, especially families, can be jerks and you can either let it bring you down, or you can remain positive in spite of them. You certainly want to make sure that your children see how you handle grace under pressure because no amount of gifts or money will ever teach a child how to have grace, class, decorum, and inner-peace. Don't fret about things you cannot control...taking from the AA creed, here's a good prayer to remember: "Lord, help me to change the things I can change, accept the things I can't change, and grant me the wisdom to know the difference."
Finally, from a practical standpoint, I wouldn't rely on others to supply you with your baby's material needs. While it's socially expected to give expectant parents baby shower gifts, you really shouldn't rely on anyone (even family) to get you big ticket items like cribs, strollers, car seats, etc. even if they're purchasing them for other members of your family. I personally think it's a crime to pay top dollar for any of these items when you can simply get them for a fraction of the cost at a consignment (second-hand) store. For anyone who scoffs at using second-hand items, consignment stores in particular don't accept poor quality items because their business is to actually SELL these items. Garage sales in high-end neighborhoods like Hyde Park and the nicer areas in S. Tampa are also a good idea...you'd be surprised how many yuppie moms actually never use their baby items and sell them at consignment stores and/or garage sales. I always feel like I've hit the jackpot when I find unused, unwanted baby goods. My pride is not so great that I'll turn down a fantastic deal when I see one. Clearance items at Wal-Mart, Target, Baby Depot, and even dollar stores are great places to save a buck or two on baby clothes.
My husband and I are ready to welcome our third child come this September, and we've already scoped out the bigger items at consignment stores...our favorite is Baby-Go-Round on Henderson Blvd. in S. Tampa - 4111 Henderson Blvd, Tampa, FL 33629 , Ph: ###-###-####. We're not having a baby shower prior to my due date...instead, we're going to have a post-delivery "welcome" party where we, as a family, can take inventory of what we really need. Because we're getting most of the bigger items ourselves and babies really only need replenishables like onesies, diapers, and wipees for the first few months, we'll end up getting something more useful than physical gifts: hard, cold cash. :) I was wondering, do you have a lot of friends who live close to you? Don't forget that your friends aren't exempt from baby showers...in fact, it's your closest girlfriend who should be planning and fretting about all the details on your behalf.
Blessings to you and yours.