I'm still sleepy, so hopefully this will make sense...
First, don't worry about the board. I think it's natural for people's level of involvement to fluctuate depending on life events & even daily schedules. If I don't have enough time in my day, I'll be off the board and trying to get another hour of sleep, or hanging out with the boys.
Second, everyone makes --and is responsible for--their own choices. You, your sister, the rest of your family. Your choice is to eat differently, and you are taking responsibility by stocking up on your food. They buy food that they choose to eat as well. It sounds like you've offered to share with them as well.
Third, naps are important for babies. All babies are different and some are not as naturally adaptable as others. I was a young mom with DS1 and the only one of my friends with a baby so he ended up going everywhere with me & sleeping whenever he got tired. I never worried much about naptimes, or schedules. DS2 didn't do well with that, so I needed to keep him on a schedule --for the both of us. He was a happier baby that way. DS3 came along and was really an easygoing baby so I reverted back to what I did with DS1. I had people that were critical of my parenting styles each way. Either I wasn't giving baby enough structured schedule, or I was letting him run the show. It's not their decision how to parent your child. You are doing what you need to do, and that's enough.
If any of your family doesn't like waiting for DS to wake up for a nap, they could go ahead and leave you to catch up later. They have the freedom to make those decisions for themselves.
Suggestion for you: I tend to forget sending cards as well. The only time I do well is when pre-buy a large package of mixed occasion cards and stamps and keep them handy. Then it only takes a minute to send. It might not be the 'perfect' card for that person, but at least it goes out.
I think the fact that you're so worried about being self-centered probably means that you're not. I've known several truly narcissistic people, and they would never worry this much about it. If you're still concerned, ask DH to assist by stepping in if he's concerned about your behavior. I think you'll be just fine though.