Barcelona players will be additionally motivated by Real's match result, because they play earlier today than Barcelona. To some extent, it may indeed motivate Barça players, but I think they are already in a very good shape to beat Real Betis. You are right to point out that the latter are in good form, but you haven't paid attention to their performance. If you look not only at their performances in the league, but also in general, you will notice that they are unstable in this respect, but more often than not they score little. What's telling here, you can see it even from their goal difference within La Liga
Barcelona need to win their match for at least maintaining three point difference with Real Madrid or better expanding it to 5 or 6 points if Real Madrid can not win their match. Barcelona will feel sort of pressure if Real Madrid play first and win first, and playing under pressure contains more risk. Barcelona will hope a bad result from Real Madrid, but they have to be clear with their task is beating Real Betis and getting three points.
With their outstanding form recently, Barcelona are able to beat Real Betis even this opponent is fighting very hard for their chance to get a fourth position. This ambition of Real Betis can create enough challenge against Barcelona in this match.