How do i balance helping my sister (26F) with her mental health + my relationship with my GF (24F)?

I (24M) just got a condo and was planning to spend the night there with my girlfriend. I’ve been making it a point to be more intentional with the time I spend with her, because we’ve both been busy and have missed each other as of late.

My sister and I still live in our original home, which we plan to sell later this year. She’s been struggling with her mental health a lot (the family has been trying to convince her to seek an official diagnosis) and has asked me to stay at home with her so she’s not alone. She said she didn’t wanna chat, but wanted to feel my presence there with her.

The whole situation makes me worry that in the future, one of the two relationships closest to me will give out. I’ve decided to cancel plans with girlfriend and stay home with sister, but I fear for what will happen if this pattern keeps up. I love my sister dearly, but it’s cutting into the time/effort I’d want to spend on my relationship with my girlfriend.

For anyone in a similar spot, how do you cope and what’s your advice for balancing the two? Much appreciated.