my mother is threatening to hire a lawyer because we do not allower her to see our kids. should we be worried about her winning?

There is also an issue as to whether mother and father are together.  If the parents have separated and have been separated for 6 months or more, or have started divorce proceedings, she may be able to get "some" custody.  Typically, a court may give grandparents an overnight or two or maybe a weekend a month in a "normal" situation.  If there are significant safety concerns, it may only be supervised custody, if anything.  However, if the parents are still together, she does not have the ability to seek any form of custody, even visitation.     If, however, your question is whether she could get primary custody or majority custody, that is doubtful if her only complaint is that you do not allow her to see the children.

Generally, grandparents have not custody/visitation rights unless they qualify under the specific subsections of the grandparental custody and visitation statute. You should consult with a lawyer to see if granny here meets that criteria. Summarizing the key point of trhe statute and how it relates to your case, granny would have had to been a primary caretaker for 12 months or had been more or less acting like a parent. This happens when in cases when mom and dad are recovering from a drug addiction and granny takes the kid in. When the parents are sober again 12 months later, granny now has rights. If she has no rights you can tell her to pound salt. Just joking, try to keep the peace.