Jaffacakes, to me it sounds like you have done everything you can in this situation to give your husband, his daughter and the grandchildren space to be together and that you've endeavored to be polite and kind without being overbearing or solicitous. I would also have a difficult time being made to feel unwelcome in my own home. I'm guessing that the feelings of resentment simmer silently more and more as time goes on. You probably will not be able to change anything at this point so, if it was me, I'd do something along the lines of what Namsnanny suggested:
quietly and with as much dignity as I could muster, serve the cakes and biscuits and say something like Well you two go on and enjoy your time together, with my blessing, but I'm going out/watching tv in the other room now because I am not included. By being genuine and non confrontational, and taking control of the situation, you will have shown her (and of course your H) that you recognise this treatment as bullying. By withdrawing from the situation you have shown self worth.
Decades ago I was in a situation that I knew wasn't going to change and the hurt and indignation I was continually subjected felt like a slow burn. I decided on a short and polite but direct communication of my thoughts and feelings and felt much better afterward! Maybe if you didn't feel like you had to swallow your own feelings, going off to read a book in the other room or taking time with friends would be a fun diversion. As a side benefit, both your husband and stepdaughter would be made very aware of the reason for your absence from their company in each future visit.