While Robert Dudley, Earl of Leicester, was the second most painted person of Elizabethan England after the queen (and a corresponding number of portraits of him survive), we do not have very many portraits of his brother, Ambrose, and his sisters, Mary and Katherine (in fact, there seems not to have survived any portrait of Katherine, Countess of Huntingdon).
Mary Dudley, in 1551, married the courtier Henry Sidney, and it is possible that a full-length portrait of her by Hans Eworth was intended to commemorate this marriage. There is also a miniature of Mary Sidney, painted by Lavinia Teerlinc in about 1575.
Robert Dudley’s elder brother Ambrose, who received back the family title of Earl of Warwick in 1561, seems to have shown a certain resemblance to his father, John Dudley, Duke of Northumberland.
Mary Sidney in the early 1550s
Mary Sidney c.1575
Ambrose Dudley, Earl of Warwick
Links:
http://www.nationaltrustcollections.org.uk/object/486275
https://collections.vam.ac.uk/item/O174799/portrait-of-mary-dudley-lady-miniature-teerlinc-levina/
All Things Robert Dudley 
Are there any of Lord Guildford?
There are only surviving portraits of Ambrose, Mary, and of course Robert Dudley. Guildford was still very young when he became “king” and he didn’t “reign” for a long time, as we know. (There are no coins of him or Jane either.) There are several 19th century fantasy portraits of him, though.
The RD portrait of him dressed in red is Ambrose, you can tell by the arms. On RD’s arms there is always a sideways crescent, which shows him as the second son with Dudley arms, it’s cadency in heraldry. The man in red doesn’t have it, it’s Ambrose, the senior brother, so the full Dudley arms holder. Then look at the face, then cross-reference other portraits called RD. Most portraits called RD are AD.
And I consider the portrait above, called Ambrose Dudley, to be John Dudley, the brothers’ father. Daddy Dudley. Tudor portraiture is an absolute mess, with massive errors that supposed experts to not have the confidence, eyesight, or insight, to correct. You may say — who am I, know do I know? I’m the only person to look critically over the entire field, afresh without preconceptions, and then can mouth off as to what’s right and wrong without fear of losing some career in the field.