Q: My husband and I are trying to deal with his ex-wife and are so frustrated we don't know what to do.

He and "M" were divorced in 1976, after 10 years of marriage and two daughters. She was involved with another man. My husband was willing to forgive and continue the marriage with the help of professional counseling, but she would not.

My husband finally put the past behind him, leaving the Pacific Northwest to come back east, where we met.

We have been married four years and have two sons. Our relationship is excellent although there were some rocky moments until my stepdaughters and I worked out some rough spots.

Our problems are financial: We feel stretched to the breaking point.

My husband paid far more of the girls' expenses than he was required to until we married, and then his ex-wife sued for the "status quo." Our lawyers worked out an out-of-court agreement, with concessions on both sides. While we pay considerable child support and medical expenses -- our share and more -- she repeatedly abuses this agreement and threatens legal action to get more money if we question anything.

We don't want to be hauled into court. We're not afraid of losing but we fear the terrible expense of dealing with courts thousands of miles away.

The girls lose most because the antagonism strains our relationship with them and the expenses limit their visits. We can't afford the $1,000-plus it costs to fly both girls east at once, so they come in alternate summers. This year neither came.

"M" refuses to share expenses for these trips -- but offered to lend us money at 15 percent interest. To her, money is power. She isn't hard up. She has a good job, but the girls look like paupers, in outgrown, ill-repaired clothes.

I question "M's" concern for the girls. She works with the public schools, with summers off, but since they were toddlers, she's had them spend the entire summer with anyone who would take them -- family, friends, camps.

I am about to encourage my husband to file for custody. It would mean problems, especially with the 13-year-old, who is plagued by physical and emotional conditions and has never done well in school.

How can we support the girls best?

A: It's always a mistake to do the right thing for the wrong reason. That's why you have to sort your feelings with ruthless honesty.

It's galling to pay more than your share, but don't think of it as money taken from the boys. This only makes it worse.

If you think your stepchildren are getting poor care and that you could do better -- and if you think the girls would be happier with you and their father -- your husband is obligated to file for custody. Only ill health or an inability to nurture four children, of such diverse ages and background, should stop you.

However, if you're buying into "M's" game, using money and children as weapons in a battle between families, you'd better let things be. Custody cases are treacherous unless they are absolutely necessary. You might save some money in child support but the court costs would be major and the day-to-day tension of living with teen-agers you barely know could take a high toll. All of you deserve better than that.

Whatever you decide about custody, however, you can't let the ex-wife bully you indefinitely or she'll just keep upping the ante.

If her state and yours have adopted the Uniform Reciprocal Enforcement of Child Support Act (and most states have), she will probably sue under those terms and the suit will be transferred to your district. This would be cheaper for you and if she comes east to fight it, she might even agree to mediation instead. Here an experienced professional would referee them in face-to-face encounters, keeping the dialogue on target until both parties reached agreement.

During all negotiations, it would be better for you to stay in the background, enlisting some outside support for you and your husband. Among the best: the Stepfamily Association of America (28 Allegheny Ave., Suite 1307, Baltimore, Md. 21204), which has a quarterly bulletin and self-help groups across the country; Step-Motherhood, a realistic book by Cherie Burns (Times Books, $14.95); and Remarriage, a fine monthly newsletter for $30, published by G & R Publications, 648 Beacon St., Boston, Mass. 02215. In it, you'll read about other stepfamilies with similar problems.

Whatever you do, keep in weekly touch with the children and try to see them more often, even if your husband flies to them. A greater closeness now will make them feel welcome at your house when they are grown.

Questions may be sent to P.O. Box 15310, Washington, D.C. 20003.