How do You say you are my brother from another mother in Welsh?

There isn't a direct equivalent in Welsh for the phrase "you are my brother from another mother." However, here are a few ways to express the sentiment:

Formal:

* Rydych chi'n frawd i mi, er nad ydym yn berthyn. - "You are a brother to me, even though we are not related."

* Mae gennym ni'r un gwaed, er nad ydym yn dod o'r un fam. - "We have the same blood, even though we don't come from the same mother."

Informal:

* Rydych chi fel brawd i mi. - "You are like a brother to me."

* Rydych chi'n frawd i mi, yn ysbryd. - "You are a brother to me, in spirit."

* Mae gennym ni gysylltiad cryf, fel brodyr. - "We have a strong connection, like brothers."

Humorous:

* Rydych chi'n frawd i mi o fam arall, neu efallai hyd yn oed o dad arall! - "You're my brother from another mother, or maybe even another father!" (This is a playful way to say it, and it implies that the person is very close to you.)

Ultimately, the best way to say it depends on the context and your relationship with the person. You can choose the phrase that best conveys your feelings.