25yo, found a lump in her breast, grandmother, aunt on fathers side had cancer, advice pls?

Please See Doctor. Many breast lumps found on (self) examination are simply "lumpy-bumpy" breast tissue. If a dominant mass is present, it could be a cyst, a benign tumor, or a cancer. The best way to differentiate these is to see a breast surgeon and/or undergo evaluation at a breast center. While it is highly unlikely to have breast cancer at 25, it is not impossible.
Only to get checked. With that family history, should not put off getting examined. Likely her gyn can facilitate u/s or mammo and possible needle biopsy to see what is what. Call gyn & good luck!