What your mother did was not illegal, however, the highway would not be a great place to pick children up. Obstructing a highway passage is illegal but it really depends on the highway.
If she owns the property she can deny entry; though it's probably not the safest idea in the world to be picked up on the side of a highway. You might want to calmly and coolly discuss the situation with your mother at some point to figure out why she wouldn't let the boyfriend on the property and see if anything can be done to improve the familial relations. Best of luck
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