I don't think it's weird to take just her out. You could take stepdad out alone on Father's Day. If you were having a general family dinner I would agree you should invite stepdad, but Mother's Day is her day.
When is Mother's Day in the UK? I've seen a lot of threads about it, but here it's not until the second Sunday in May. Is it a different day for you, or are you all just more prepared than I am?