The 1851 Census has children Robert 7 (1844), Margaret 5 (1846), Elinor 4 (1847), Jane 2 (1849)
The 1861 Census has children Jane 12 (1849), Elizabeth 8 (1853), Catherine 7 (1854), Margaret 4 (1857)
Jumping ahead and assuming Roberts/Williams marriage, 1843, Ffestiniog (Merionethshire) is a possible, possible birth registration with mother's maiden name Williams is Jane Roberts, Sep Qtr 1849, Ffestiniog (Merionethshire).
Finding others Roberts who match ages on '51 and '61 Census, mother's maiden name Williams and Reg district Ffestiniog proving somewhat difficult. There are possibilities but collectively they don't hang together.
PS - I know absolutely nothing about DNA!