Your use of the term 'gas station' (and the time of your post) suggests that you're probably in the USA. This site is based in the UK, where our laws are very different to those of the US. So you're unlikely to find anyone who can answer your question here.
I suggest that you post again, on a US website, such as one of these:
http://www.answerbag.com/
or
http://answers.yahoo.com/
(It might also help if you say which state your brother-in-law is in, as state laws - rather than federal ones - might apply).
In the UK a witness would only be obliged to attend a court (and to testify) if a witness summons was issued by a court. The USA refers to our 'witness summons' as a subpoena. I suspect that a similar situation will apply. (i.e. your brother-in-law can't be directly compelled to attend by a lawyer but can be compelled by a court order). However contributors to the US sites, referred to above, should be able to provide you with a definitive answer.
Chris