Hi there.
My DH was a widower. I have two stepkids; when I met them they were 11 and 13. They are 14 and 16 now. I also have a DD with DH now.
I can only tell you my personal take on this, realising that there is no one 'right way' to handle it.
I have always taken the view that my role (vis-a-vis memorials and remembrance for the kids' mother) is to facilitate but not to organise or participate. I have on occasion suggested something to DH, and I try to think ahead to events or times that might be tough for the kids, since my DH is not always great at foreseeing those things, and talking to DH about what they might need. When we moved in to our new house, I encouraged him to sit down with the kids and choose some photos of her (and other family) to display; they took over the kitchen and looked through old photos while I read in my room. I have made sure DH saves things of hers for when they get older. At the holidays and meaningful dates, I try to give them all an opportunity for some space and time together. At Christmas they have a candle lighting tradition, like ohchrist mentioned above. She is not buried near where we live, so there are few opportunities to visit her grave (something DH and the kids don't attach that much meaning to, anyway); when we were visiting the area, DH took the kids to see the grave and they had a nice day together while I relaxed and played tourist, then met up and heard about their day.
However, I would personally not ever introduce a new remembrance tradition, nor would I participate in one with them. I feel strongly that this is a part of their life together which I should not insert myself into. If the SCs had been younger, maybe I would feel differently. I think the kids and DH would feel constrained by my presence at these times, and it gives them an opportunity to remember and reconnect as the unit they are. I don't feel excluded by that anymore, and I don't feel it is my 'job' to do it, either. They have friends and relatives who actively work to memorialise their mum, and that is as it should be, but that is not my role in their life, and shouldn't be. I am always there painting the scenery, I guess, but DH is the director.
As time goes by, and DD gets older, I imagine she and I will continue to do 'our own thing' at times like this, while DH and the SCs do theirs. This Mother's Day (DD is 21 months), I am thinking that I will take DD on a day trip and have a bit of a mummy-daughter outing, while DH does something with the SCs, then we'll all have a nice evening together and watch a film or something. Again, if they had been younger, maybe this would be different, but under our circumstances, it feels like the right way for us, and the way that respects everyone's feelings and place - including my own. Well - it works for us!
Sorry if this is a long reply. Hope it helps.