To wonder why on earth someone would do the following

[NeedsAsockamnesty]

Walk into a room pick up a baby that was not theirs and change its bum and clothes when they had been done within about 10 mins and genuinely did not need doing.

Every single time you came into contact with the same baby?

And no it's nothing to do with a mil

No idea. If it was my baby I'd tell them not to change the nappy and clothing as I'd just done it.

A show of being in charge and suggesting the Parent/Carer isn't doing a good enough job.

Who's doing this and what do the child's parents say about it?

I would wonder why said baby's mother or father didn't step in and say "Oi, what are you doing changing Little Freddy's nappy?" and wrestle the whelp away.

Because some people enjoy changing babies the freaks and if it's how they interact with them and the baby is happy then I'd just let them crack on, no harm done.

Don't invite them Over?

Teenaged baby mad niece or similar?

I would ask them what they're playing at if it was my baby.

I'm guessing MIL.

ask them to pay for the wasted nappies

Family member with no children? Or one who hates you?

I'm guessing SIL.

I'd wonder why on earth the parent was allowing this person to pick up and change their baby ?

If you genuinely know this person and that is what they do why don't you call them when the baby does need changing and ask them to get on with it?

Or perhaps your baby smells and you just don't notice it .

Is this going to be a massive drip feed?

Do they have a poor sense of smell and believe the baby has a wet nappy then carry on because they have courage in their convictions?

Do they want to prove to other people in the room that they are responsible / would make a good parent or carer?

Do they have little experience of kids yet think this will help you?

Are they seeking to undermine you?

Whoever it is, you need to stop them and say "you don't need to do that, I changed baby's nappy 10 minutes ago and her clothes are clean." If they do it after you've said that then you need to set them straight about you being the parent not them etc. But I wouldn't just sit there and let them do it.

Unless the person has some responsibility for the baby at that point (i.e. nursery manager), then this is very odd behaviour, and should be firmly challenged.

I'm waiting for the back story.
I'm assuming that the delivery man from DHL doesn't change your baby after every parcel he delivers

Just explain it properly, OP.
Who's doing it and why do you think they are doing it?

Need to know who it is!

This is so tiresome.

Yes it's weird.

Who is it?

It's my sister, there is no way the baby smells in any way, my sister has little to no experance with babies. she's also not the helpful sort but she's not spiteful either (I do have one who is) and she does genuinely like the baby

It's just boggling my brain.

At the moment I'm sort of well it's not hamful or damaging the baby isn't bothered by it, but I'm tempted to ask her why just because it's odd.

I did once say ive just done that, and she replied with "oh I don't mind"

The only person I knew who did this was the mum but she didn't have custody and only saw the baby on contact visits.
ALL she knew how to do was to change a baby's nappy and dress it.
So she did.
Every single time regardless of need.

But I doubt that is what is happening in your OP. So tell us already!

Need more context

Who is doing this? Are you the parent allowing this to happen?