Looking after young kids is exhausting for anyone of any age, so I say ‘hats off’ to your husband for caring enough about his son and new partner to help and support them. It’s probably very difficult and tiring for him especially as he’s doing it without any practical or emotional support from you. Even though you find young kids boring can you not find it in your heart to be forbearing with good grace for the sake of your own relationship? There must be something, some activity, that you enjoy and could introduce to and share with your step-grandkids, and so get to know them as individuals and grow a relationship with them.
If your stepson’s new relationship is to have a hope of surviving the couple and their blended family will need practical support from time to time over the years. If you want to be a full part of this family, and nurture your own marital relationship, you must play a full part in this, within the limitations of your work life. After all, the childcare is not every day and there is still plenty of opportunity for you and your husband to live your own lives.