Is SIL so desperate to upsize she's unintentionally conning MIL out of money - venting but very worried too

[being40]

My MIL has just come back from a holiday with her daughter (SIL).
My FIL passed away in December 2018.
MIL is still very much grieving. She has been with FIL 54 years. He was ill and she nursed him at home.
MIL lives 3 hours drive from us and 1.5 hours from SIL how we until FIL got sick it was DH and myself and our DC who spent every holiday with them. SIL not estranged from MIl just didn't see her as much.
That's the background.
MIL is staying at ours and told us that she was thinking of selling up her flat and helping SIL buy a 'much bigger house' and moving in with them.
It turns out plan was mentioned by SIL on holiday as MIL is feeling really bored and very lonely and unhappy (understandable).
But the plan is way more advanced and SIL put her house up for sale as soon as they got back and has just had an offer. MIL says sale could take months (when DH and I said she might need to think about it).
We are very concerned that SIL didn't even call DH to mention plans. They used to be close but since SIL married her DH she 'ghosted' both DH and I.
Oh and MIL has told us she wants to pay for us to have an extension so she can move in with us the rest of the time she is not at SIL.
Other warning signs to me: SIL selling propert for £650k and buying a £1.7m one. They are in late 40s so her DH is taking in a huge mortgage and they want to turn garden of property they have found and put a offer on (already) into glamping site.
Not much I can do but same thing happened in my family and didn't end well.
Just venting really and wanting to be supporting of grieving MIL.

Thank you - a lot of useful advice.
I'm not worried about inheritance- MIL helped us buy our house and we had planned an extension as we have an outbuilding which we were going to renovate. MIL's money welcome but was not expected.
I'm worried at the speed of all this and the fact they are upsizing so much.
Said to DH 'why don't they find a place that's got some extra room for your mum' without her having to put in any extra cash. Or somewhere like ours that they can extend.
MIL plans to split her time 40/60.
Thing is SIL - is not a natural carer and although she has 2 DC - they spend most of their weekends with her MIL/FIL she joked she 'wants them out the way as much as possible'. Admitted she found looking after children hard. But she will need to help MIL because although she is able bodied she is in her 80s...

I think the daughter is taking advantage while her mother is vulnerable

Those of us who have seen these things go wrong aren't concerned about inheritance.

MIL will have sunk her money into a large property for her and SIL.
SIL can make all the promises in the world but if she fails to care for MIL in the future or makes her feel very unwelcome, 40/60 could easily become 100/0 with OP and her family. It very much affects OP's family, or could do.

And there might not be the funds for a nursing home if MIL ever needs that kind of care.

Given MIL’s age she also needs inheritance tax advice depending on the size of the estate.

Oh and MIL got really angry when we tried to sit down and ask her if she had taken legal advice

I wouldn't be committing to 40% residency to someone who wasn't prepared to sit down and have an adult conversation about the situation.

You may need to reframe it. Tell MIL she is a wonderful and generous person. Say it would be a dreadful shame if her kindness was undone by technical tax and social care concerns. Point out that it would be very stressful if this was challenged later so she should get some independent legal advice so make sure that her intentions don’t get messed up by the complex rules in this area.

I agree with @ajandjjmum.

@ChazsBrilliantAttitude's post is good advice though.

Unfortunately MIL didn't want to chat about this at all. So thank you for all your advice. She actually started a tantrum when we talked to her claimed we were harassing her. I ended up having relationship with DH compared to SIL with her DH - they are much calmer and speak to each other really quietly.
We will have our extension done I think as DH and MIL want that.