My gf and I do something like you describe with your sister in law. She and her husband have a two year old, I don't have kids or a primary partner. On an average month, I:
- watch her son two evenings and occasionally a weekend day
- hang out at their house for dinner two evenings
- go out with her on a just-us date once
- spend time with her at social events (dance parties, kink parties, etc.) where we get some quality time once or twice
I feel like the time I spend babysitting makes up for the time I take her away from her husband. However, the only reason this works for me is because I genuinely love her son and want to spend lots of time with him. I've known him since the day after he was born. If it weren't for the fact that I genuinely wanted to bond with him, I probably wouldn't be up for watching him nearly as much.
Also, even with that, the limited just-us time we spend together places our relationship strictly in the "secondary" category -- it's not a central aspect of our lives, even though it's important to us.
For context, I also see a couple of other people, and this all balances for me quite well.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.