Melissa Etheridge -- Tammy Burned Our Kid with a Cigarette

Melissa Etheridge has just filed legal docs, claiming her former domestic partner, Tammy Lynn Michaels, accidentally burned one of their kids with a cigarette.

Melissa, who is in an epic battle with Tammy, wants the judge to order both her and Tammy to go smoke-free around their twins ... to prevent any cigarette-related accidents from happening in the future.

In the docs, Melissa also claims Tammy has accused her of having no maternal instinct and therefore is against striking a 50/50 joint custody deal. Melissa says Tammy is adamant ... she did not give birth to the children to give them away.

Melissa calls Tammy "angry and vindictive."

And highlighting yet ANOTHER display of nastiness, Melissa says Tammy once sent her an email saying, "You have to support so many households because YOU BROKE SO MANY HOUSEHOLDS, silly. If you had decided to have one ounce of integrity/honesty as a person or partner, you would NOT be in this predicament. Think about it."

In her docs, Melissa attached an email she claims Tammy sent her way ... which says, "They aren't leaving my care while you are on the road -- a completely inappropriate place for 3-year-olds -- not to mention you can't track them, have no child care, and they were bit by dogs in the house last time you had them."