the isssues are 只 and 应该

You can explain it is not 只习惯, but without other info, 亲戚 is more believable. Just look around in China, which big company is 只习惯相信自己的亲戚?
The problem is 只习惯相信自己的Party.

你应该喜欢表妹，因为她是你表妹 means you both comes from simular family education, simular mind set. Plus if you can get along, both your and her parents will be happy...