MIL Advice :(

So my fiance has a 3 year old daughter from a previous relationship. Long story short on her, his mother acts like the world revolves around her. Literally. She gives in to whatever the child wants and doesn't care about anything other than her. The kid is at our house 25% of the time, MIL house 25% (cause I'm pregnant and can't watch her as much) and at her moms house 50% of the time. When him and I found out I was pregnant, my main concern was that me and the baby wouldn't be his main priority and I don't want my child picked second... Anyway, got through that, I'm now 21 weeks pregnant, and the other day we went over to MILs house to pick up his daughter and she Sat us down and told me that I need to include the kid in more things cause I'm always posting on Facebook about how my fiance and me and baby are a little family. Anyway, it just really hurt. She basically attacked me and the only thing my fiance could say was "Oh it's not her kid." The daughter is not my child and never will be. I don't like the kid cause she's so horribly behaved but I watch her and love her and discipline her when necessary. Isn't that enough? I don't know. I'm just sick of my MIL not caring as much about this baby as she should. And I need some advice on what to tell her or how to handle it.