I just don't understand how people can be this way.

I've always had issues with my husband's family, it's been non stop. I don't make problems but they always do something that just leaves me shocked on how people can be so mean. I can tell stories for days but I won't. I have two children. One is a bonus child (11), who came from a previous relationship of my husband's and the other we share (3). My in-laws only watch my 3 year old if it's absolutely necessary and I have exhausted all other resources and its what they've wanted. They've never had a big interest in my daughter and I'd rather not force a relationship and have her treated poorly because I pushed them to take her for visits, sleepovers ect anyways, my mom is my best friend. She was just diagnosed with two different cancers, one seems to be fast progressing and the other we aren't sure yet... she needs chemo/radiation. All her appointments are rolling in and she will be seen by the cancer doctor next week and get the information on when she starts treatment, where it will be and other information regarding her cancer... but because her family doctor is in another town where she was diagnosed they wanted her to be seen by the specialist near her location now as traveling while doing chemo and radiation isn't necessarily ideal. Anyways, I took it hard. I mean who wouldn't? Shes my mom, my best friend, my daughters entire world. Life without her just seems impossible and it's like I have a rock in my stomach. I held myself together while we got the diagnosis and I gave her positive reassurance that it will be OK. I couldn't cry because then she would cry more. I knew I was going to break down and I knew I needed to allow myself to get it out privately because I have to be strong for her... I reached out to my MIL and asked if she could watch my daughter for a bit and I told her why... I told her I just needed to cry hard and scream a little because it just seems unfair and it hurts me I can't fix this for her. I know my daughter is old enough to understand feelings but she's not old enough to understand why I'm having these feelings... I'm numb but sad at the same time... It's not fair to her. She said no problem... next thing my SIL is messaging me acting like it's a huge inconvenience for my MIL... as she was picking my stepson up for the night I guess. Made me feel like I'm horrible for asking this of her then said she would take my daughter instead.... so I said no.. I didn't know it was a problem...she could've said something and I wouldn't have been mad. I don't know why they have to talk behind my back and why my SIL has to control everything. She's a horrible person and I'll never ever have her watch my daughter because she's horrible to her... like over my dead body. She was instantly messaging my husband on me asking my MIL to watch my daughter for a few hours like I was totally out of line to do that. I don't get it, I don't withhold my daughter but I also don't force them to take her just because they take my stepson... they know the doors open whenever they want to be involved. It just seems so cruel.