How to adjust with having stepkids?

I'm 29, no kids. My fiance has 2 daughters. 1 is married, 22, other one is 19, being co-parented. Youngest stays with her dad every other week. So we are currently in a long distance relationship, still waiting for my fiance visa to get approved. Any tips on how to adjust if my stepkids would be really close to my age? Any tips for changing my perspective of wanting him all to myself? How do I learn to share him with his kids?
How to adjust with having stepkids?
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