I get along better with my friend’s sister than her how can I become friends with her without my friend getting upset?

So I’ve been friends with this girl for around a year or so now. We’re not super close but we hang out once every week or two weeks and catch up. I get along with her pretty well but sometimes I feel like we don’t have that much in common.. she’s 8 years older than me. Her energy is often abit draining.. like she always looks at the negative side of everything. So, last time when we went out for dinner she brought along her sister and two nieces. Her sister is closer to my age. We got along very well and actually I felt like there was more of a connection, we have similar interests… hobbies.. we are both students.. and even our view on relationships is very similar. We even have the same style😂. Her sister also works night shifts that’s why she can’t always go out. I feel like we could become bffs but it’s kind of weird to start a friendship with her without my friend feeling left out. Mind you I have very few friends.. I prefer to have few close friends with a real connection than a lot of shallow friendships.

How can I navigate this situation without my friend feeling offended or left out😅. The thing is most of the times I am the one who initiates everything… and when we do meet up I sometimes feel drained.. not because she is a bad person but because she went through a lot.

I feel like if I were to hang out with her sister alone it would be weird but I really think we would get along so well.

I get along better with my friend’s sister than her how can I become friends with her without my friend getting upset?
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