Haven't had time to read the whole thread as I'm about to go out but didn't want to read and run -
My exP moved in with his new DP about 6 months ago. He is rubbish with our DS (6) but his new DP is fab - she does his homework and reading if he has any while he's there, bakes with him, takes him out on daytrips etc etc.
I accept that as a mum it is really hard to see someone else having that sort of relationship with your child and you can feel quite territorial and jealous. Although, as a PP poster said, it's also important to get a grip!!!
I love that he has someone when he visits his DF that bonds with him and does all the things I would do with him. It is beyond me why anyone would begrudge their child having another adult in their life who loves them and looks out for them.
I know some mums are funny about nail varnishes etc but as you've already said DM is fine with this then I really wouldn't worry.
If she does have a problem with it then that would be her problem, not yours.
My DPs both remarried wonderful people after they got divorced and I feel as though I have four parents rather than two. It didn't take away from how much I love my mum and dad but my DSM and DSD have both done so much for me and never made me feel like a burden and I love them both to bits too. Children have no limit on the amount of love they have to give and I think parents need to accept that when their exPs meet someone new rather than letting jealousy get in the way.
As an aside, if DS complained to me that his DSM had told him off the only thing I would say to him is to respect the rules of the house he's in and that she would be well within her rights to do so if he was breaking those rules. If I thought the telling off in question was unfair, I would say nothing at that point to DS, but would talk to EXP to establish what had happened and if it was unfair I'd say something to him about it. When you're together it's important to put on a united front to children regarding discipline and I don't think that changes when you're not together either, there's just more people involved and normally two separate set of rules to enforce so it becomes a bit trickier.
To me, you sound like you do a wonderful job with your DSD and I don't think you should worry about stepping on toes. I think it would be very unreasonable for her DM to feel that way 