Did I go about asking over for dinner the wrong way?

Dear Miss Manners,
My sister-in-law invited me to a Pampered Chef party she was hosting. I called her to thank her for the invite, and let her know that I would not be able to attend. While on the phone I then told her that I would like to have her and her husband over for dinner. I asked her to get back with me and let me know what day would be best for them. It has been almost a month now and they have not returned my call. I mentioned this to my husband and found out that his mom told him that the reason his sister did not return my call was because I went about it the wrong way. That I should have given her a date and then asked them if they could make it on that date. That I should not have asked them to choose a day that was best for them. I thought I was being nice. Was I in the wrong?

Confused