My DSS was 6 when I got pregnant with my first and we told him after the first scan. He was so excited to become a big brother but he'd been asking for a sibling for some time and he loves babies. How is your DSD about babies? Does she like them?
DH told his ex when I was 8 weeks gone to give her time to get used to it and forewarn her incase DSS needed support when he was told. I think it's respectful but they have a good relationship.
Her only reaction which was a bit unexpected was she requested a list of the savings etc my DH has in place for DSS. I can see her point as she was looking out for her child's interests.
Then our DD was born on DSS's birthday and she went a bit mad about that and on DD's 1st birthday. But apart from that one meltdown, it's been fine since.
DSS wasn't jealous but he did regress a bit - lots of baby talk, wanting Daddy cuddles, climbing on me when I was feeding the baby etc.. It lasted about 3 months.
He would also get tearful about going back to mums and missing his sister but we see him a lot so he coped well with it.
He did struggle with disturbed sleep to begin with and was tired in school etc. but that's not a stepchild issue, it's just what happens when a baby is in the house!
2 years on, we have 2 DD's and DH's ex had a DS. So DSS went from no siblings to 3 in a very short space of time. He loves it!!
Your PIL - I'd park that concern to the back of your mind. Don't worry about a problem unless there is one and if, when the time comes, they are disinterested, don't worry because you will love your baby enough for all of them.
I think you are very young but I became a mum at 32 and it hit me like a ton of bricks so that's my view. It's such a lifechanger they I was glad I'd had my 20's to myself.
If you feel ready and you've done the things you wanted to do before having kids, then no one can tell you it's not the right time.
Your parents will be fine and if they're not, I bet the baby will win them over!
Good luck.