Hi fluttercub, first just to say my advice comes from a kind place as I have been there with the same concerns as you. It's really hard to on one hand to be expected to love and care for and integrate your family with your step children, then on the other not get too close for fear of offending or confusing them. It's almost like you are expected to turn your emotions off and on completely at the whim of others.
However, you are not legally anything at all, other than you husbands wife. You have no legal rights at all in regards to the children. For that you would need to have Parental Responsibility. That is sometimes agreed for step parents if, eg, mum has passed away, or is completely absent. Some people choose to have step parents as PR. I'm thinking of it for my own dd as her sm often takes her to the doctors etc. and it would probably be sensible.
The very best thing you can do now is detach. I don't mean altogether. When your step children are with you you provide a loving environment for them. As though you are a kind aunt or loving grandparent. It's great that your child accepts them too, that will add to the experience of a loving family.
As far as your child goes, don't make any promises. Don't tell them about a visit until it actually happens. When my dsd went no contact for six months my dd suffered terribly and even had play therapy because of the issues. My biggest regret was forcing (or rather over encouraging) her to believe her step sister was a big and important part of her life. In reality that wasn't my decision to make and when mum alienated my DH, we had no power to stop it.
I had encouraged the "this is your sister" because I thought it was the right thing but in these high conflict scenarios it is not a good idea.
In regards to the amount of contact. It is pitiful. What is your husband doing about this? My husband would be banging on the door of a sols office, not passively complaining about a text message that hasn't been returned. These children deserve a relationship with their father and he is failing them by not fighting for this.
The best thing you can do is keep a happy, healthy, fun home environment. Welcome the children when they come. Let your DH provide maintenance money to mum, and tell him if he wants to appeal for decent contact with his children then you will support him emotionally all the way.
I know you feel that you love these children but your responsibility is for yourself and your own child. If/ When he/ she starts suffering as a result of all of this you will realise where your love really lies.
Good luck.