I think the fact that it's taken them almost 2 years to actually get pregnant perhaps you should just enjoy this time for them and get more involved. It's really not an issue in the great sceme of things. Women have been having babies for millions of years, it won't stop just because you are pregnant. I have found that it's nice to have another friend or family member to enjoy it with and talk about etc. Not worth worrying About!
Hello,Here's what I know.
I was the first child of my parents' to get pregnant and so I was excited to have the first grandchild. Then, 6 months later, my brother and his fiance announced they were also expecting. I felt jealousy and resentment. I worried my baby would be pushed aside once their's was born because he would be 6 months old and no longer that squishy little newborn that most people prefer to gush over.
But what I didn't know was how close my son and his first little cousin would grow to be. They are now both 12 years old and the closest of friends/cousins. They adore each other and I adore their relationship. I'm SO THANKFUL they got pregnant when they did because, had they not, my son would not have this lifelong best buddy that just so happens to also be his first cousin.
As for your SIL's partying hobbies- give her a chance. I know lots of people (my younger brother included- not the one I mentioned above) who had some pretty atrocious extra curriculars but went cold turkey the moment he found out he truly was going to be a father and is now one of the most loving, involved parents that I know.
Hello,My cousin's wife was pregnant with their first and due a month before my long awaited second. There was a lot of talk about doing stuff together and our girls being close friends because of how close they are in age. None of that happened though. And while we did have some comparisons I pointed out the differences as well. Lots of times people just don't know what to say so they just say whatever random nonsense pops into their head, it's not really got anything to do with you. My fall back response has always started "My doctors said ..." to help remind people that I have educated professionals whose opinion I trust and whom I would consult if I were worried about something.
Even if you weren't preggo at the same time as someone else who is close to you, you'd still get comparisons and ignorant comments ("whoa you're huge, are you sure there is just 1 in there"/"you are tiny, are you sure you are pregnant and didn't just have a big lunch").
I'm sure the other mum wants her pregnancy to be special as well. I wouldn't worry too much about her encroaching on yours, but if it did start happening or people started talking about having a combined shower, I'd politely let them know that you think each baby deserves to be celebrated in their own right. Or, for a non-confrontational approach, just say you aren't available any of the days they try to schedule something combined...