Every year since my daughter was born my mother in law claims my children.before now I was told I could not claim my own children becouse she paid the bills... every year she says I cant but she can a

You need to answer a few questions before an answer can be provided.

Do you and you children live in the home of your mother-in-law?  If so, how long have live with her?

Do you work?  If so, what is your income?

Are you married or single?

you can claim them if these 5 test are met for the qualifying child(ren)
1) related to you
2) under 19 at end of year and younger than you or spouse if MFJ
or
under 24 at end of year and a full time student (enrolled in a qualifying school during any part of 5 calendar months in the year and younger than you or spouse if MFJ
3) child must have the same principal residence as you for more than 1/2 the year (temporary absences like away at school are ignored
4) the child did not provide over 1/2 their support
5) the child didn't file a joint return - exception if filed they filed a joint return to get back taxes paid in when there was no tax liability
now lets say all 5 tests aren't met then you have to check to see if person is a qualifying relative by meeting these 3 tests ( relative is a misnomer)

1) member of household - lived in your household all year except for temporary absences or be related
2) gross income under $4,150 for 2018
3) you provided offer 1/2 person's support.

thus if you meet all the qualifying children tests, the fact you m-i-l provided over 1/2 their support is irrelevant

if you don't and she provides over 1/2 their support see can claim them if she meets the other two tests for qualifying relative

if both of you don't meet the required tests, neither of you can claim them.  

if you qualify but she has claimed them and already filed, you'll need to paper file.    eventually the IRS will send each of you a notice asking you to amend (to not claim dependents)   if no one responds the IRS sends a second notice to each asking for proof.