[on] Company asks to return early from Mat leave (18months)

I am taking 18 months mat leave to be with my baby. It was 12month originally but extended as I could not find a childcare and also I wanted to be with my baby longer.

I am middle leadership role (director/senior manager) and my team member resigned and as such HR is asking to return only. Could I say no? The thing is as my team member resigned I probably need to backfill her role which is not my responsibility before going on Mat leave. It’s quite stressful:/