Hmm... do you want her to be portrayed as "nice" or as a, well, the bad guy in Sess/Kag relationship?
I love all the others had mentioned above, then I remember a story that told SessMama and Inu no Taisho's background story wonderfully. Try reading
The Lady of The West by see03, rating M so no YIMs. The SessKag part is on the last part.
The other one is
Second Alliance by Imani Joain, rating MA so no YIMs either! I love this portrayal. She's both a loving mother (as loving as she could be) and ruthless. Deadly combination, and a total badass, I must say. The story has a dark theme, and sometimes could be disturbing... but it is addictive.
Bitter Like Chocolate by Midnight Song (rated MA, no YIMs!) is another one, very good, the drama is addicting too. You can't help but crying for more chapter on this one.
A light, adorable one:
The Adventures of Chibimaru and Gang by MissTeak. Chibi Sesshoumaru is always adorable.
His Retainer by JeniNeji is also a good one. I think it's so cute to see SessMama and InuPapa together. And I will never see Jaken the same way ever again. LOL.
Or you can check out Kanna37's
A Winter Wish (MA, no YIMs), or
Windows on The West (MA too), and if you're looking for a bad Mama, read
Bitterly Yours (MA too, of course no YIMs!!). And for a light one:
Welcomed Interference