I don't want to leave LO overnight yet

No way. Not with that dog.

No not unreasonable and you have proven it is not an issue with trusting her by agreeing to a day trip. She should be happy to have this opportunity, why would it be more important that he spend time there asleep? Babysitting is usually looking after little people who are mostly asleep while their parents have dates. An afternoon is quality time.

My advice is to agree your boundiaries now with your husband and stick to them so you are both always on the same page.

I think you are perfectly reasonable with this. It is not reasonable for them to expect to have your little one stay at the moment. Young children sometimes become unfamiliar with people they may know well but have not seen for a short while. I saw my eldest grandson on an almost daily basis from when he was a baby and he was very used to me and stayed in my house from about a year old.
When he was about 20 months I went to OZ for five weeks. When I returned home he was unsure of me. I made no attempt to invade his space and as other family members were there we chatted and riffled through my suitcase -initially he would not accept a gift I had brought back - then suddenly he got down from the sofa and came and hugged me! - things were back to normal.

Mine didn't stay overnight a another person's house until they were about six.

They didn't want to and it wasn't necessary.

Do what you and your child wants. Nobody else counts in this equation.

Lesim91 I don’t think you’re being unreasonable. An aggressive dog and a small child shouldn’t be under the same roof. Based on that alone it would be a non starter for me.

You’re definitely not being unreasonable. What’s their plan for the dog when you’re LO is at their house moving forward? That would be my no 1 concern.
You know your son best, you’re his mum so know what he can cope with, so keep being a good mum and putting his comfort and safety first.

Absolutely no way should your little one stay away from you overnight until both you and he are happy to do this.
In the circumstances with covid and an aggressive dog that is a definite no anyway.
My youngest daughters mother in law wanted DGS to stay with her from weeks old and got very cross that DD and son in law wouldn’t let him stay.He is two and still hasn’t stayed with them.
The first time our DGD was to stay with us,DD and son in law missed her so much they came to get her!She is four now and stays with us regularly,her choice.
It’s hard but don’t be manipulated.There will be plenty of time for sleepovers when he is older.Your baby,your rules?

You are not unreasonable.
The dog is probably very jealous and therefore potentially dangerous as he may lunge and bite. Best avoided.
If your MIL is always unhappy then you will never please her....nor do you need to.
Stick to your instincts and do what is right for you and your son.

No, you sound very reasonable.
Let her take it however she chooses to, and continue to do what's right.

I'm surprised she would want to have a one year old who barely knows her for the night.
I'd run a mile at the thought!

To me the dog would be the deciding factor. No way go along with it.