I was alone with two kids for 10-11 hours a day and never even received a text from MIL
Did you text and ask how she was doing? You say she's busy, so perhaps it didn't even occur to her.
Lockdown was a very challenging time for everyone.
I agree with pp that she probably doesn't want to interfere and be accused of being the MIL from hell.
I know that my DM is able to be free, easy and say what she wants to me and Dsis, where she wouldn't with SIL, for fear of upsetting or appearing critical.
When it's your own children, you have a bond and know their not likely to take offence at the slightest of things.
You mentioned that your MIL offered to move in with SIL depending on the restrictions when she had the baby. I know that my DM is more comfortable when she stays with me or Dsis in in comparison with when she stays at my brothers.
Also, equal treatment and closeness are two different things.
So your MIL may end up closer to the other GC, but she can still treat them equally.
You said your MIL has a good relationship with your DD and I think that Lockdown impacted her seeing your DS much.
I also don't think geographical distance is not necessarily a huge barrier to having a closerelationship. I live in the same city as DHs parents and mine lived 200 miles away when the DC were younger, yet our DC saw my family more often than the other GPS...who were only about 18 miles away.
My DC are closer to my parents and I notice that when they say Grandma or Grandpa, they always mean my parents, but when referring to DHs parents, they would say Daddy's dad or Grandad [surname]
A pp also said something I absolutely agree with and very often, it's the DIL who makes the effort over her DH to see his own DM. How much effort does your DH make with his mum?
The number of threads I've read on MN where MIL visits and the DH suddenly finds all the jobs he needs to do and won't stay in the room with his DM, leaving DIL to entertain her.
I know that I have to almost push DH to visit his folks, so naturally my DC are closer to my parents and all the Aunt's and Uncles on my side of the family.