AIBU to be annoyed about MIL coming to toddler class?

Not yet ;) I adored my granny more then my mum and my mum was never jelous. Parents have too many resposibilities, worries etc. Grannies are more in the moment, relaxed during play time, no expectations. At least the good ones.
This connection is precious and so beneficial for your child. I would be so happy if that was my MIL.
As a parent you have your child 24/7. And during playtime its natural that the child runs to others and only proves you are a great mum, that it feels safe to be away from you.

Can’t you all sit together as a family? I can’t see that the MIL is “muscling in” or being a “horrible woman”-she is going with her daughter. Are you expecting her to push your DS aside and not interact with him? Seems very weird to me!

I'd go to a different class but just give a blasé "oh we just didn't feel like attending last week / month" or such like. They seem to be a couple of people that will thrive on the drama if they smell a touch of one brewing so don't give them an inch. Don't ever tell them where the new one is either.

Can you say that as you see them at the playgroup you don’t need to see them again in the week.

I’m glad I’m not the only one who thought about cashing in on the situation 🤣

You have the patience of a saint if you are already seeing that horrible woman every week!

She is taking YOUR child, not her daughter's child, she's trying to push you away. It's weird, you are the mother, what is she trying to achieve

Find another class.

are you the overbearing MIL then 😂

In your situation I too would find another class, and/or switch to a different time.
I can see why you're annoyed, you've paid for this class for you and your DS to enjoy together, and mil and sil have essentially taken that away from you and your DS.
Neither one of them sound sensitive or understanding of other people's feelings.
I'm sure there will be other times that your DS and your MIL,especially as she seems rather pushy, can visit.

Why are you blaming your MiL? She is at the class with her own DD and has every right to be if your SiL invited her.
It's also hardly her fault your DS wants to spend time with her. I'm sure you'd have a problem if she wasn't the sort of grandma he made a bee line for.
Its not U to not want to be in the same group as them. I understand it changes the dynamic.
Find another class, either instead, or as well as this one.

Sorry i am beeing blunt. But thats how i see it. SIL and MIL are warm and connecting. Kids love that. You are obviously not. So work on it, as its sad for your son and no wonder he runs to other adults.

Do whatever you need to do to protect your enjoyment and bond with your little one. Don't let yourself be pushed out of your own family life. I have a very similar MIL who elbowed me out of my children's joyful events, in the nicest way so that I didn't recognise it until it was too late and the traditions had been set. I'll always regret not fighting back harder and earlier. Being a mum is so much work. You need to have the joys too, and it's very hard when someone else appropriates them and leaves you with just the hard parts.

Take advantage of your MIL taking an interest in your child and go to another class with just your child and you.

You don’t have strong connections with your in-laws, but you and SIL both have similar age kids and your MIL is making an effort.

You could have a lot worse to complain about. Eg they don’t bother with either of you at all and she has no interest in your child. I had a nasty MIL I had to go no contact with.

Also if you’re continually having negative thoughts toward them both, they’ll feel that off you. Do you want healthy relationships for your child in future with their family? If so take it that it’s nice they come along and get to know them and be nice. We had none of this with my DHs side of the family. My kids barely know any of them.

Honestly, I think this is quite nice. Ds gets to see his granny, who he clearly likes, and his cousin. You could sit next to them and chat and build a stronger family relationship. I don't think it's deliberate at all, or if it is then it's deliberate in the sense of "we should try and get a bit closer to OP as we seem a bit distant". Maybe because my in laws couldn't give a toss about my children and I feel a bit sad about it, but I'd love to have been able to watch them build that bond with their other set of grandparents.
However, different strokes for different folks, you clearly don't like it, so I'd stop that class and find another one that you also like so that you get that one on one time with your little one. You can vaguely just say the time stopped working for you etc.

Oh, what - so the OP can sit there like a lemon by herself every week for a joint activity she's paying for and wants to enjoy with her own son?

OP - YANBU.

Definitely ask if you can move to another class time/day.

And it's ok for them to have family time with your DC without you being there. My DDs are 20 and 17 and have a lovely relationship with PIL, their cousin and SIL because they spent a lot of time with them without me or DH being there. But obviously a lot more time with us at home. I am still their mum and that will never change.

I always saw toddler groups as a way to meet other mums and DD to be amongst little friends. If grandma tags along then SIL is less likely to mingle with other mums and grandma is taking your child's attention away from the whole point of going.

Is this a particular toddler group, rather than bog standard one in village hall or library? Trying to think why they would travel to a different town for it.

Even so, who’d want their MIL rocking up?

MIL knows exactly what she is doing here, power play.

What's wrong with MIL taking DC to the class though? It's bloody annoying that they just muscled in, but it might be a case of "let them" this time. As long as MIL will pay for it of course. Perhaps she could look after him one day a week? That's what my MIL did and took DD to a class. Then find another activity to do with DC. And you get a day off to yourself. You could perhaps turn this into a win win.

You sound paranoid/insecure. It's highly unlikely your SIL and MIL have joined a baby class you attend to try and push you out, it's more likely that they're trying to connect with you.

Givwn your updates, it does seem like they are muscling in on your quality time with your DS. I'd (very reluctantly) quit the ckass, find something else to do and not inform them.

If they pull the same muscling-in trick again, you'll know that you are going to have a very real problem with them.not acceptung your boundaries, and that will require an action plan and your DH recognising that there is a problem.

Why are people such dicks?!

I don't think toddler classes are for the mother to bond with her child - I thought they were for the child to have fun with other toddlers, or if they are babies, for the mothers to have other women to speak to!

This is your time to make friends with other mothers,I would go somewhere else,you need that independence from family,find people who share common interests,especially over the top mil who may you feel inferior and insecure. You need to gain confidence.