First things first- it would be ideal for your sister to not delay planning for pregnancy any further since she is already 30plus and also has a fibroid.
Next Steps
A 3D ultrasound or a MRI Pelvis will help delineate the exact location and size of fibroid and if it is affecting the cavity of the uterus.
Health Tips
As far as possible fertility specialist would want to avoid a surgery since your sister has not yet borne a child but it is equally important to know the merits of other conservative procedures like Uterine Artery Embolisation. Waiting could be another option, provided it does not increase her risk of infertility and/or complications during pregnancy.