You have already received lots of wonderful advice, but I will add my 2 cents. I know you are very nervous, as many first time moms are. Most of your worries, though, have fairly easy solutions if you look at them one by one.
1 - You can stay indoors out of the cold. Take a warm pram suit and hat to cover him if you do need to go outside at all.
2 & 3 - Baby doesn't like car...Ours HATED the car (crying every second) but we had to keep up with getting her okay with it, as it is a fact of life, so now she doesn't particularly care for it, but she is alright with it. Two friends have avoided the car as much as possible because their infants didn't like it and as the babies grew they never got used to it and still hate it, curtailing a lot of possible family activities. Your husband will just have to stop whenever your little one needs to nurse or need a stretch break to be cuddled. You can stop at all the tourist spots along the way and your little one should enjoy all the new sights and you'll get a kick out of how cheesey all the tourist traps are. We have been traveling with ours since 2 weeks old (Palm Springs) and continue to (Vegas, Pismo Beach, Texas, San Francisco, the Cook Islands...10 hour plane flight at 6 months old) and have found that the less you take and the more flexible you are, the better! Just take a sling (works great if you don't want strangers trying to touch your baby), a couple toys (they really just need you as their main toy!), a blanket to roll around on the floor at your brother-in-law's home, and a blanket to cuddle up with, diapers, wipes, teething ring that can be chilled (use a ziploc filled with ice) or other teether they enjoy, clothes, a ziploc of possible meds (infant tylenol & motrin, Mylicon, diaper rash cream, nose bulb syringe), your doctor's phone number, and you're ready to go. We don't usually even take a stroller as our little one is not the biggest fan of it and would rather be in the sling or Baby Bjorn. If you cosleep, no need to worry about sleping arrangements; if not, you can usually request a crib ahead of time if you stay in a hotel or just take a pack n play. Nothing more needed! One last thought, you can move your baby up to the convertible car seat now and it may help him tolerate the car more. Since ours hated the infant seat so much, we got the Britax Marathon very early on and she is much happier. It sits up higher so she can see out the window, even though she is still facing backwards, and it sits a little more upright so she doesn't feel like she is strapped down in a sleeping position and it has more leg and arm room for her to kick her legs and move her arms to play with toys.
4 - Your husband will need to stop whenever your little one needs to nurse and you can take your pump with you. Your friends and relatives will understand that your little one needs to eat often, as their children were infants at one time. I find that when I am on our excursions with my husband there, I tend to be able to nurse our daughter even more than usual as I am not worried about cleaning the house, running errands, and cooking dinner, etc.
5 - If someone is sick, you can have them stay away from the baby. You can also, very gently, ask your friends and relatives to wash their hands (they will understand as they probably worried when their children were little). Because you breastfeed, your little one has a strong immune system and a few germs will not be the end of the world. It is actually good that they are exposed to a variety of settings while they are little to strengthen their immune system and help avoid allergies later in life. (I am also in the healthcare field.) For example, my little one, now 7 months, has been going with me once a week since she was 2 months old to work for an hour a week at a school where I work with a group of kids with disabilities and I remind the kids that they can not touch my baby unless they wash their hands.
6 - You can stay in a hotel if you are really worried. Your husband can make sure, ahead of time, that his brother's home is well cleaned and vacuumed before you go.
Last part - you need to support your husband in his relationships with his family as I'm sure he supports yours with your relatives. It is wonderful that he wants to share this time with you and to show you and your baby off to his friends and relatives. You do need to continue doing things you enjoyed before you had your baby to keep your marriage thriving and it is good for you little one to experience new things, be loved and adored by the relatives, and live with a Mommy and Daddy that are happy with each other and with their life. I know it doesn't seem like it, but you can do almost anything with your baby that you did before you had him, with just a few accomodations. People are always commenting how cute and happy our little girl is when we are at places that people generally don't bring infants to, but I believe it is because we have been doing it from the very beginning and are as relaxed as possible about it; babies are VERY smart and pick up on how everyone is feeling about a situation and display similar emotions to their parents' reactions.
Good luck and enjoy your first of many family vacations! As others have said, take LOTS of pictures!