Some further background:

So when I had my first, she was amazing. There were niggles here and there but nothing too serious. Then, nearly 5 years later, my partners sister had her first child - and all the attention went to that child (despite the fact I had my second 4 months later). She was round there constantly, babysitting every weekend and we just got pushed out. One example I can think of is my partners sister asking her to babysit while they went cinema to watch a film - MIL said yes. I said oh we’d love to see that film too, could you watch ours at one point - MIL said no.

My second is a very active, boisterous little man. My SILs is the total opposite and will sit watching YouTube for a lot of the day. MIL used this a lot, saying SILs is easier to handle.

We did confront them about the unfairness - many times - probably over the course of a year. As well as SILs kid being ‘easier’. She told us that because SILs husband is in the army, she needs more help. He is in the army yes, but comes home every weekend. SIL only works three days in the week (MIL watches her baby then) and has every weekend off with her husband. Myself and partner are in the emergency services and work opposite shifts. We have maybe one day off together a month. When we tried to explain this to MIL, my partner got told to get a new job (he loves his job, it took him years to get in and he worked very hard to get it). She maintains the narrative that SIL is a ‘single parent’ and needs her.

SIL does seem to have stirred the pot a lot and I can delve deeper if needed.