I hear what you are saying definitely.
I adore my DSD and as far as I am concerned I have another part time DD (we have DSD every other week for the week). DH feels the same way about my DDs, although they are with us full time.
DSDs Mum however, makes things very difficult. She hates the close relationship that I and DDs have with DSD (and even the relationship that DH has with her I think). She cannot accept that I am effectively another parent.
I only work PT so I can collect and drop off all the girls from school. She told several Mums at school that DSD hated the joint residency arrangement and that she was very unhappy and unsettled, and that DH was selfish for inflicting it on her. It has taken months of diplomacy with the Mums for them to accept that I am not Cruella de Vil and that DSD is genuinely happy with arrangements as they are.
I actually bumped into one of her closest Mummy friends when I was out with the girls this morning and she said in a VERY shocked voice, 'Oh it's DSD, doesn't she look happy' which I think says it all about what DSDs Mum has been saying about us.
She complains that DSD doesn't get the individual attention that she does with her, failing to realise that DSD has 2 sisters here, so doesn't need an adult sitting next to her 24/7.
It is tough, you never get any recognition for anything good you do, and if you make a small mistake, or just do things differently then you get slammed, but at the end of it al, my relationship with DSD outweighs all the crap that comes with the situation.