For giving my sister the new baby gifts I received instead of the new one's?

I would have given the New stuff to my sister and passed dc1 baby stuff to dc2. That's what most people do. I think you could have given her the New stuff for her first baby so she could pass it down to the second (if there will be one). But your sister was rude to call you out on it. I would have said "Thanks sis but I prefer getting new stuff for my baby." Gifts should not be expected. So basically I think both of you are in the wrong..

This ^
I would be very offended. The right thing is - you give her a gift from you and then you offer her your old items which you don’t need anymore but not as a gift but just because you don’t need them anymore if she doesn’t want them - fair enough it’s up to her.
Your baby and her baby are going to have a lot of differences in life due to having different parents.

But it won't be unused, the OP has said she will use them for the 2nd baby, as intended by the givers. It just means she has items from the first baby now spare.
Handing on good used baby items within the family is perfectly normal surely?

Would you seriously look your friends and family in the eye, at a baby shower thst they had organised and you hadn't asked for, and say, oh , thanks for the gifts but I don't want them so you can take them away?! That is outrageously rude!!! If they had asked and OP had made a gift list then yes, that would be pretty greedy asking for unnecessary items. But it sounds like these were surprise gifts.
And turning around and re gifting the new items would be barely any better unless you are sure they'd never know.
I mean, surplus consumables like nappies/wipes/creams etc maybe. But re gifting in front of the givers is rude

It sound to me like the sister is resentful that the OP has inlaws that are either wealthier or more generous than her immediate family and she's Jealous/judgy for her marrying into a 'better' family.

You would lack any basic manners if you did.

I didn’t think people had 2 baby showers. Don’t you typically have a much smaller event the second time since you were given a lot of stuff the first time?

But why is the sister entitled to the gifts that were given to the OP from her own extended family and friends. It is not the OP's fault that her sister didn't have a baby shower. She has given her a load of stuff that was presumably still in good nick. There's nothing wrong with second hand and the sister sounds ungrateful.

And as for "you accepted them"... I mean, do you actually go around rejecting gifts from people if it's not something you are in immediate need of? Are you saying this is what the OP should have done at her baby shower?

I read all the way down expecting your inlaws to be the ones offended you were giving away gifts! Your sister is ungrateful, you have items from your first child that are available thst you have kindly offered, it would be very insulting to your in laws to give away the new gifts and keep the old ones.
She can either get over herself or go without.

She's not entitled to the gifts that were given to you. If she doesn't want the used stuff she doesn't have to take it. It was nice of you to offer. Why is her baby entitled to the things that were given to you for yours? She sounds incredibly childish.

Why don't you give her a mix of the new and old? What sort of things are they? I'd have given the new to be honest, knowing she didn't have many gifts and it's her first baby.

I feel like there’s a backstory of sibling rivalry and jealousy. I’d be very surprised if this is just about a few baby gifts.

The gifts were given to you for your baby.

I would be grateful for any regifted items from my family.

Ooh with the update that she attended your shower but won't get one of her own - ouch. That is incredibly unkind.

You don't need those gifts at all yet you accepted them, pretty wasteful and poor for the planet?

What's the backstory here, are you close to your sister, do you get on well or do you compete?

I'd be pretty hacked off if my sister took a load of new stuff she didn't need and then said "here, have my old shit for your PFB". Not very loving or kind. More showing off / lording it. Sorry.

She's being entitled. Those gifts are for your baby. She doesn't get a say.

Dose she know that the second set of new stuff were gifts you didn't expect? If she thinks that you had decided the first stuff wasn't good enough to re-use, so brought new instead, then I can see why it could come across as "this isn't good enough for our baby, but fine for riff-raff like yours".

Better left on a shelf unused?

If I had taken the time to choose a nice gift for someone's new baby I'd be a bit hurt to find out they'd immediately given it away.

So she already has gifts from people? She just wants more? That puts it in a very different light!

Has she always been entitled?