In basic conversational terms, there is no difference. You could say any of the following: "My mum arrives tomorrow." "My mum is arriving tomorrow." "My mum will arrive tomorrow." "My mum will be arriving tomorrow." Hope that helps!

This is the difference: We usually use the present simple - arrives - when a future event is part of a fixed schedule. We'd say 'arrives' if there's a formal timetable, such as "The president arrives at 7 am tomorrow. He has a meeting with the finance minister tomorrow morning and then leaves on the 2 pm flight to Paris". We also use the present simple for fixed dates such as "My course starts in May 2019" or "The special offer ends next week". For this reason, we often call it the 'timetable' future. It's not so common to use the present simple construction for a family visit unless this is part of a larger formal schedule. We would generally use the present continuous in this context, as it's a future arrangement that isn't part of a fixed timetable. "My mum is arriving tomorrow" would be the most usual way of talking about an impending visit from your mother. This is a fixed arrangement - for example, she has already booked her ticket or made a reservation - but it isn't part of a formal or regular schedule. "My mum's going to arrive tomorrow" is another construction which you could also use. This has a very similar meaning to the present continuous in this context.