Hello. welcome to HCM.
If the lump is felt separate from the testis proper, it is possibly a paratesticular lump. Epidydmal cysts are common causes. However, there are uncommon more serious causes which need to be ruled out.
it is best to get a scrotal ultrasound
(high resolution at a good centre) as it will help pick up site of lump and also define its nature.
family history of cancers seems to be unrelated. Usually most testicular cancers have painless enlargement of testis itself.
Hope this helps