My DH is the only biological child of his DP. My MIL had a DD (DH's half sister) before she married my DH's father. My FIL has adopted her though. But then when my FIL divorced my lovely MIL, first he had an affair with a woman who had a DD from previous relationship. FIL ended that relationship and then married a woman (who passed away 8 years ago) who had a son from a previous marriage. So my FIL has 4 children out of which only 1 my DH is his own biological child. Quite complicated.
So when he dies, his estate will be divided 4 ways. However, for my MIL, it will be divided of course, 2 ways between my DH and her DD (my DH's half sister).
I don't think OP is being entirely unreasonable on this. Also, I have 2 DSC. DH and I are planning to have one of our own as well. When we do come around to write up our wills, it will divided equally in all of our children.
I can understand if they don't want to include you (given you're not married) but your daughter should be part of it. I agree with one of the post here about treating your step child equally in all aspect is what is more important and fair.
You should talk with your DP. You have been together thick and through for 10 years. He has played an important role as a parent to your DD too. Leaving her out, how does that make him feel?
I do agree it is not something you should bring up with your MIL but you have every right to speak about this with your DP.