I had my first DD as a teen, it was the right decision for me, I had always known that having children was important to me.
I then had problems and it took ten years to have my second. So, whilst my DH and I had a good life together, there was a bit missing, for me.
I had my second and third in my late 20's/30's.
I then re qualified and have the career that I came to want, I am so pleased that I didn't do my BA in my first choices, it would of been a waste.
I have had the life that I want and will always continue to do so, except for being Widowed young.
My eldest DD is your age and her DP is pushing to start ttc.
She doesn't have the "Maternal Urge/Drive" and she likes her lifestyle of Festivals, Weekends away, Beauty Treatments, being able to leave the cleaning/dishes etc, which would change.
She would have baby sitting on tap, I do the same for my Niece. But it having a child does put a dent in income, freedom etc.
You have to somewhat enjoy what children bring to the mix.
If you haven't got the craving for a baby, then don't have one. Just be honest with your DH.
Tell your SIL straight, no-one should talk down to you.
I was on the other side, I've had to tell my career wanting Sis to cut the remarks out, I wouldn't of had her life if it had come gold plated.