Brother and SIL and DM fall out

[MersyMersy]

Need some advice. My DM,B and SIL have had a falling out and I'm seeing my SIL tomorrow and I don't know if I should mention it or if she's going to mention it I don't know how to react.
They had a falling out over my DN at the weekend and I've seen some texts my SIL has sent and they're shocking. I'm actually quite angry at how she has spoken to my DM but she is my friend and I don't want to seem like I'm stirring things up.
My DM had my nephew at the weekend so that they could go out for their anniversary. Apparently they didn't go to sleep till 8 in the morning, turned up late to get him still drunk, vomitting and staggering everywhere. My DM was upset about it and told my B so. Then she got text messages from my SIL saying that she (my DM) was out of order, that my SIL was angry at her, and that my DM should have expected them to be hungover and how dare she criticise them for having a good time! I'm disgusted that she has spoken to my DM like this. There was more but that's the general jist of it. My B apologised to my DM but she obviously doesn't feel the same way. My DM is deeply upset because she was meant to babysitting today and they didn't show and she's worried that they will stop her from seeing my nephew. I personally think that getting smashed till 8 in the morning when you have a one year old to look after is pretty poor regardless of Wether they are getting babysat for the night. WWYD?
Apologies if this post reads rubbish I'm doing on my phone!

I'd stay completely out of it. If sil brings it up, sidestep with a 'thats between you, db, and dm'. And the same with your mum tbh.

Tbh it sounds like there's a huge amount of exaggeration going on here.

Be careful.

I would also stay out of it.
Presumably if they were too drunk to look after the child/drive then your mum would not have released the child back into their care?
And to be honest, it's good that they're having a good time whilst their child is safely with a grandparent - lots of parents of 1yos are contemplating divorce with all the upheaval having a little dc brings. Dh and I certainly were, perhaps a night out getting drunk would have been beneficial.

Agreed. I really hope she doesn't mention it at all tbh and text messages are assumed to be private after all. My SIL is quite fiery tho and she has tried to be derogatory about my DM to me over other things. Its just hard after seeing the texts, I am annoyed with her. Just got to zip it tho I guess.

Idespair- I totally agree that they were entitled to a night out too they have a lot of problems latey which is why my DM agreed to have my DN in the first place. She didn't want to give him back and had them stay for dinner until they sobered up a bit. I think they didn't end up going home till late. I think its all got a bit silly with the texting. My DF was also upset and he has been texted too. I just think its madness.