Mae eich mam chi mor dew, bod mae ei math gwaed hi'n siocled!
[edited for corrections]
Almost correct in the first part, "Felly" means "So" as in "It was hot so I took off my shirt" whereas you would use "Mor" for "It was so hot." (Roedd hi mor boeth), note that mor causes a soft mutation. I'm not sure what you mean in the second part though.
I was going for "her blood type is 'chocolate.' (I think this is the first time my urge to make a 'yo momma' joke has also been a learning experience!)
Hmm okay well I've never heard "Blood type" in Welsh so I'll use your "math gwaed". I think the sentence would be "Mae eich mam chi mor dew, am ei fod ei math gawed hi'n siocled" but I'm not totally happy with my translation.