You do have an interesting situation.
There are definitely other step-moms here who, for all intents and purposes, are their step-kids' mothers. (I am not one. Although my 11-year-old step-son lives with us and his mother lives across the country - so he spends a lot more time with me than with her - that has only been the case since he was 8 and he is very clear who his "Mom" is.) However, I can't remember anyone else mentioning that their step-child's mother is deceased. I'm sure that casts a very different glow on the relationship!
On the one hand, you don't have the worries a typical custodial step-mom has: conflict from a resentful ex-wife and/or resentment from a child who may feel you "stole" her father from her mother, or that you're trying to replace a mother who is still in this world.
On the other hand, most of us whose husbands are divorced derive some sense of clarity and security from knowing our life partner made a conscious choice not to be with his previous wife anymore and to be with us instead (in whatever order and time-frame those decisions happened...). Your partner's continued respect for his deceased wife's wishes must encourage you, by showcasing his capacity for loyalty and enduring love. At the same time, you must wonder occasionally: "What if she were still here?" And, of course, there's a natural human tendency to idealize the dead, whereas you can't idealize someone in the same way, when you live with them day in and day out.
I understand what you said about not feeling like your older step-kids are really your step-kids. My husband has an older son, who was in college when we got married. I think he's wonderful, but he feels like more of a "family friend" to me, compared to his younger brother, for whom I wash laundry, cook meals, drive him to sports practice and volunteer at his school! Also, I get along well with the older son's mom and I feel squeamish emphasizing the word "mother", in my relationship to him, because I recognize that I did none of the mothering of him, only she did.
As for your religious situation... and I thought ours was complicated! My husband and I are both Catholic and raise all of our children in our faith. But my husband's ex-wife was raised Jewish, by parents who adopted her at birth. She became estranged from them, in her teens and early adulthood. During that time, she reconnected with her (Christian) birth mother and married my husband. She doesn't practice any religion and generally seems content to observe the secular parts of whichever holidays people around her celebrate. She agreed to let my husband raise their son Catholic and did not expose him to anything Jewish until their divorce. But since then, her birth mother passed away and she made up with her adoptive family and neverendingly flip-flops about whether she's Christian or Jewish, in a manner that's maddeningly superficial and unmistakeably aimed at perpetuating the conflict with my husband. When she had custody, suddenly my step-son became "Jewish" and people "weren't allowed" to talk to him about Jesus anymore. Not that she took him to Temple or taught him anything about Judaism! She just wanted to take away that bond of religion he had with my husband. Now that he has custody, every other year she sends a photo of a little Christmas tree or gingerbread house in her apartment and sends my step-son an "Easter basket" instead of a "Purim gift basket", to bolster her argument for having him on Christmas Day. But when she has had him then, they don't celebrate Christmas, but Chanukah with her parents, as usual. It's so clearly a ploy to make sure my step-son misses Midnight Mass and the big kid-fest on Christmas morning, at our house. Then again... religion's such a tricky, subjective thing that there's always a little, nagging voice in the back of our heads, saying, "What if, this year, she really is going back to Christianity? After all, she is dating another Catholic guy, now. Is it right to deny her Christmas parenting time?...." It's a mess.
In short, I understand your frustration. It's a nice gesture to raise your step-daughter according to her late mother's wishes. But if neither your partner's home nor his mother's are Jewish homes, what really is the point? How can she possibly be learning that religion has any meaning deeper than holiday traditions? I respect your restraint in teaching her about your own beliefs, when she's already in a confusing environment. Perhaps you could research the ways in which Jewish and Christian holidays wound up being scheduled around older pagan celebrations - and how some of the traditions overlap - and share that with her, when she's older? You might be able to show her the history and influence of your own beliefs, without negating what she is taught about her mother's and grandmother's. After all, the fact that we're not certain when Jesus' birthday was and that it was convenient for early Christians to celebrate it around the time of Saturnalia - and the fact that we've co-opted certain aspects of celebrating nature into Christian traditions (bringing an evergreen inside, to decorate during winter...) doesn't mean there's no value in celebrating Jesus' birth and making it a decorative, festive occasion. Sorry, I'm less familiar with how such things correlate with Judaism. But it's interesting to study the fact that "paganism" is much more than just a new-age thing that's sprung up among young people with alternative lifestyles.
When I first joined, I had an interesting exchange about dual-religion families with another member, who's Jewish, but her husband is Eastern Orthodox (I think?). If you're interested, I think you can click on my name and look up my earliest posts.
I hope you enjoy Mothering!
One woman in a house full of men: my soul mate:
... twin sons:
(HS seniors) ... step-son:
(a sophomore) ... our little man:
(a first grader) ... and there is another female in the house, after all