I'm not a step parent so have no skin in the game but I do know several.
You do know that you're all in an invidious position, don't you OP?
Everyone in this situation should cut themselves some slack.
I actually feel sorry for everyone here being locked into a gruelling fortnightly routine of visits, travel, laundry, cooking, and people coming and going.
Realistically OP you're sharing yourself with one child. He's sharing himself with three; two of whom are much older, and live miles away.
Your DH needs downtime (as do you). Where is the space in all of your lives for quiet weekends mooching around, impromptu time with friends, getting your life admin done?
Apart from eating together there's very little that high school kids will want to do with a preschooler for any prolonged period of time. (Actually older DCs ages are relevant... twins in year 7 is a hugely different prospect to them being in year 10/12 for example).
A few people have asked you what you want OP and apart from your DH being more involved you can't say.
You and your DH need to sit down and have a really good conversation about the following:
How do you all want your time with DSC to be?
How do you want it time with DH/DC to be?
How is it for you when DH isn't there?
How are you and DH both collectively and together going to do anything other than be on this relentless parenting treadmill for her next few years?