Although I wonder if I can explain well with my English, I'll try to do it.
In this sentence:
実際 = actual (--> like "in real life")
妹 = sister
ちょっと = a little bit (--> like "at least")
俺 = me
気持を分かってくれる = understand me (understand what I feel / understand my feeling)
じゃない = a negative tense (--> like "might")
と思う = I think
*分かってくれるんじゃないかと思う = 分かってくれるだろうと思う
= I think (the person) might understand me
= If the person has a little sister in his real life, I think he might understand what I feel at least "a little".
(However, I'm not sure if my English is correct or not.)
There are many cases that we have to change the words when we translate the sentences and that the direct translations sound unnatural.
I hope this was helpful and other people explain to you better than me.