If you are close then probably they've seen each other a few times based on your SIL being newly separated and then unfortunately kept making plans in the moment.
I think it's absolutely fine to either create a WhatsApp group with them both give it a fun name like ladies who lunch and then say you'd love to make some plans with them both to maximise you mat leave.
It's very lonely with a baby before you find your feet and it's totally normal to feel like this. I remember being the same. It's just a hard learning curve. Quite often I think people assume you are busy, or tired, or doing your own thing.
Definitely make plans for yourself such as baby groups and rhyme times at the local library but also put yourself out there and issue invites and make it known you would love to spend more time with them.
I think these comments of 'you aren't her daughter so stop expecting so much' are pretty heartless. You're being left out and its hurtful even if it's unintended but also at 4 months you're just out the newborn phase. They maybe have been just getting on with things and waiting for you make it clear you're ready to get out a bit more now. So go for it. If SIL needs extra support then fine but I'm not sure that should be at the cost of your relationship with them both.
And I would suggest you find the days you can fill yourself with classes and routine first and then try to make standing plans with them.
Such as - Hi I've started a baby class on Mondays, would it work for you both if we met for coffee on a Wednesday morning or made some other plans?
I'm finding the weeks are very long with dh in work and it would be great to have that to look forward to every week.
The other option, which I prefer much less because I like to manage my own relationships, but is to mention to your dh that you have noticed they are busy together and it's upsetting you a bit. You should be able to have an honest conversation with him, he may have a bit of insight and he may be willing to have a bit of a chat with one of them and signal you'd like to be more actively involved with their plans.
This isn't an ideal way to go about it really but he also needs to accept that your support network is limited and be willing to help make sure you are happy.