My in-laws have been in the area during the winter almost 30 years, due to their advanced ages this year they didn't venture south. We have been coming to the same park that they own in now for 10 years. We own our lot and are 3 miles from the border west of Mission on I-2/US83. We are in a gated community as many are here in "The Valley". When people cross the border the last thing they do is hang around here. They head north asap, The Border Patrol catches some at check points on US77 and US281 about 60 miles north of The Rio Grande.
You will see Border Patrol vehicles just about anywhere on I-10 and south, so if she is concerned about "US non-residents" being in the area, it's going to greatly limit your options for warm weather. We do not cross the border any more. At one time we would cross a couple times per month, but not in the past 3-4 years. BTW way the weather has been great since we got here in early November, the best we've had in the past two years. The low last night was 64°, now at 11AM it's 78° and breezy. The high temps have been in the mid-80's for the past week or so.