Toxic SIL and pending visit

So how much time would your daughter spend with her Aunt?

Sounds as if the biggest problem is your husband who would let her go there alone.

Why ?

Why has it happened in the past?

ETA

Also, if you are there working how much will you be able to prevent things that you don't want from happening?

This is one of those do you choose shitty option number one or shitty option number two situations.

You have to decide what you want to avoid the most.
Either you being in your sils company
Or
Your daughter being there alone without you to intervene

Tbf all I'm reading is 1. The mum comment which was probably repeated by dd wrong it could of been a passing comment which was innocent and intended to be funny ie wow I'm like a second mum at this point due to bla bla and 2. 'They' said u looked like a corpse , yes not nice but some people have banter like that not thinking about upsetting people there just abit open with there opinions (not nice I know) but other than that has she done anything else for all this avoidance?

Seriously she has said so much stuff to try and come between me and DH and me and DD that anything she says is not without intent or innocent.

It kind of sounds to me like she believes you've 'stolen' her brother, and that if she can separate both DH and DD from you they'll go her. And she's preparing DD for it with the mum comments.

I hope that DH is made of sterner stuff, but sadly I'm not 100% convinced if she pushes hard enough.

Your MIL sounds relatively reasonable. Can you try to have a phone conversation with her and get her on side, explaining you're not comfortable with DD having a sleepover with SIL? Make it clear, nicely that if she can't guarantee that then you aren't comfortable sending DD for the week. If she goes against your wishes, go and get your daughter and bring her home.

She's also planted seeds in DD's head about her being her real mum

This is deranged. I think let them go, but warn your DH: she won't be doing a sleepover with SIL and if she says anything like that to DD again, you will do everything in your power to make sure she never sees SIL again. And if he doesn't support you, you have a serious problem with your marriage.

Stay in touch. If she goes for a sleepover against your wishes, go and pick her up however far it is.

Go and meet her for yourself and come back and tell me it wasn’t a serious comment

Well all the more reason to go with her then! You need to protect your daughter first and foremost.

The OP has made it very clear that the SIL described herself as the real or second mum, and that the child was confused by this.

So if your DH is her brother then surely SIL's comment wasn't a serious, 'I'm your actual mum', rather she felt like DD's second mum at that time. I would think maybe your DD was upset or needed a cuddle etc.

Wow @CosyLemur , yet another member of the males side of the family thinking it’s ok to be rude, disrespectful and overstep boundaries because they are “family”???maybe understand that the reason so many of us on here “bash” the fathers side of the family is because they are toxic as f*ck and see nothing wrong with their words or actions!
how is telling a 6year old they are the real mum ok?????
Also, I have no problem telling my side of the family if they have done or said something inappropriate, shame men can’t seem to do the same with their side of the family so maybe that’s another reason there are issues!

I agree, you need to go with your daughter. At least then you can see for yourself own eyes what SIL is up to, especially as your DH seems incapable of putting his daughter's welfare first.
I'm also wondering what your SIL meant by the mum comment. I assume your DH is her brother?

This!! You say yourself that your DH regresses when he’s with his family, that you don’t trust him to defend either yourself or your DD.
You’re in your 20-30s(?) and have obviously put in boundaries to protect yourself. But your 6yr old can’t do that and one of her parents needs to do it for her.
If her father won’t or can’t, then I’m sorry but you are going to have to step up and do it for her.
That unfortunately means you have to go with them and WFH there.

So your DH thinks it’s ok to leave his child with a woman with mental health problems, knowing she’s saying things to his child who can not possibly know that aren’t true…

You need to make sure he actually understands this is not acceptable. But also I would not trust him in taking dd away because he can’t or won’t stand by your wishes when your not around.

oh dear. I think unless you can stop your DH taking your daughter, you are going to have to go. I knwo it will be difficult for you but protecting your daughter is the most important thing and it doesn't look as though you can trust your husband...what that says about your marriage long term only you can decide.

I have said YABU because if your husband is as spineless as it sounds, absolutely you need to run interference and be up there to protect your daughter. So YABU not to go because you can't trust him to look after her properly.

It's obviously a bigger issue than this one trip, but since your question is, are you being unreasonable not to go (rather than are you being unreasonable to stay with him), yes, I think you are.

You absolutely cannot let your DD go and spend time with SIL.

essentially she is telling your DD she is the product of incest and the family are OK with that, the whole lot of them sound like lunatics and are to be avoided.

Id be having words with your DH and telling him how dangerous this behaviour is for your DDs mental health

She sounds deeply troubled. Of course you're not unreasonable not to want to go, and it doesn't sound suitable for your daughter either

Wow! Yet another Mumsnet post where we bash the fathers family!
What exactly did SIL say to your daughter? Did she say "I'm your mum" or was she missing mummy cuddles cause she was away from her mum and dad for a week so your SIL said "come here I'll give you cuddles like your mum would"?

If cannot reach an agreement with your husband on how he is to handle this, I feel you have no choice but to go. It is more important, at this stage, to protect your daughter. You will have to seriously consider your relationship going forward if he won't back you up.

It’s perfectly fine DH saying you need to understand SIL’s got mental health issues but as DD is only 6 he needs to understand that’s way beyond her comprehension!

I think you and DH need to agree on a way to handle these situations so you can trust him when you’re not there. This can be done in a kind but firm way, bearing in mind your SIL’s problems with her mental health.

It isn’t right that when he goes back to his parents he becomes completely passive and just goes along with what his mother says regardless of what you think (reading between the lines I think this is what you’re saying). He needs to put your family unit first.

Can I ask? Is she rude or strange to other people? Does she fall out with people or upset people...does she have drama?

I ask because if her behaviour is based in mental problems (very vague term that) then it's likely she has relationship problems elsewhere too.
If it only seems to be between you two, it is probably more calculated and personal.