Ugh. First, your brother is a bitter asshole. Secondly, he is a bitter asshole because misery loves company and as long as your not miserable with him he'll tell himself what he has to believe to downplay good own situation for himself to feel better. Ergo, thirdly, his talk has nothing to do with him or being CF or a woman, he has no wisdom to offer on this topics, but it does say a lot about him that that he needs to bring you down.
Now, I am only 30 and so are my friends and luckily they are only starting to pop out kids. But no matter if there are many people with kids in our lives or not, I believe one is in the end always responsible for their own wellbeing and can only rely on oneself to find fulfillment. It's good and important to have friends or other relations. But in the end relying too much on then for fulfillment is wrong, this goes for everybody, which is why I also think it's unhealthy to find fulfillment mainly through others like the own kids. Anyway, yes, it can be hard to lose friends and sometimes it's very bad when lots of them break away in a short time. But this can happen by anything, like moving for a job or having to care 24/7 for someone suddenly or an illness or accident or by marrying someone or just by life in general. That's why it's important to find stuff to rely on and indulge in in oneself. Some people do charity work or have hobbies or read. I study. Studying is all I ever wanted, it's something I can do without others, that even excludes others, and it makes me feel alive and in flow.
I'm only about to find out what a full life us for me. So far I've got: spending as much time as possible with the SO, having sex often, spend time with my parents and be there for them if needed, study, work, care for, invite and party with friends, drink tea and read good books on weekends, by biological and fair products if possible, do sports if I want to, cook and enjoy as much art, flying of any kind and nature as possible, and enjoy good food.
Since I started working full time and studying for my second degree, I don't have any time for all the stuff that should and could be done anyway.
You know, I don't think fulfillment will be the problem if you find a good balance of minding your values, fulfilling your responsibilities and having enough superficial fun. If you've found your mixture (which can change through life), having enough time for it all might be the bigger problem ;)