Trying to see if I can get some advise on a predicament I am facing at work right now.

Currently working in a team of 4 people including the boss.. since Jan 2025, I have been supporting this one colleague from a different location who is on long term medical leave. She is only slated to return in March 2025 although not confirmed yet. Boss asked me to assist I said okay

Now she is asking me to also cover for another staff who will be going on one week raya leave during quarter close which is a busy financial period.. so basically I will be doing her task on top of my work and it’s gonna be crazy week.

I just feel this is become too much and I’m being misused like this? Last year for Deepavali I couldn’t go on leave since nobody to back me up but now suddenly I need to back ppl up? Totally unfair. I am planning to tell it straight to my boss but wanna do it in a professional manner, any advise?