Sibling-in-Law
Sibling-in-Law
[A single layer of a family tree with 15 people depicted as sticky figures side by side is shown. They are connected alternated either by a bracket on top or a short line between them. The four outermost figures on each side are faded out in gray.] [In the middle is Cueball and from below an arrow points at him:] Me [To the left Ponytail is connected by a bracket and the arrow below says:] Sibling [To the right of Cueball Megan is shown connected by a small line, an arrow below her reads:] Spouse [Further to the left and the right the next figures have an arrow below with the nested text:] Siblings-in-law [The next connected figures on both sides are drawn with two other wider arrows embedding this statement:] Also siblings-in-law, I think? [All remaining figures left and right have similar arrows below and the text is:] Possible additional siblings-in-law???
[Caption below the frame:] People complain that “<X> th cousin <Y> times removed” is hard to understand, but to me the most confusing one is sibling-in-law, because it chains across both sibling and marriage links and I don't really know where it stops.

This comic shows the complicated way that English refers to sibling-in-law family relationships. As shown in the comic, your sibling's spouse would be called your "sibling-in-law" (brother-in-law for male, sister-in-law for female). However, your spouse's sibling is also called the same way.
The confusion lies with your siblings-in-law's siblings. Randall says they may be "also siblings-in-law, I think?" and further relations are also "possible additional siblings-in-law". 
According to Wikipedia, "sibling-in-law is one's spouse's sibling, or one's sibling's spouse, or one's spouse's sibling's spouse"; therefore Randall would be correct with the "also siblings-in-law" on the right (his 'spouse's sibling's spouse') but would be incorrect regarding the one on the left (his 'sibling's spouse's sibling' would not generally be considering a sibling-in-law).
Wiktionary lists a more restrictive definition: siblings-in-law are either "the sibling of one's spouse" or "the spouse of one's sibling". This definition includes only those whom Randall calls siblings-in-law, and none of those he calls "also siblings-in-law, I think?".  The spouse of the sibling of one's spouse or the sibling of the spouse of one's sibling are to be referred as co-siblings-in-law. If anything, this shows that the definition of a sibling-in-law is loose, justifying the "I think ?" sentence of the comic.
Many families also use the term "out-law" to jokingly refer to the distant sibling+spouses which Randall seems uncertain about.
The caption compares "sibling-in-law" to "<X>th cousin <Y> times removed". This family relationship, for example, 1st cousin once removed , is used to describe your 1st cousin's child or the first cousin of one of your parents. The "once removed" indicates that the family relative is one generation above or below yours.
The title text  describes a scenario in a traditional wedding in most English-speaking regions. Prior to the wedding being completed, the officiant will provide a final opportunity for anyone in the audience to speak a reason to object to the wedding. This intended for reasons why they cannot lawfully be wed -- such as that one of the participants is already married to someone else or is too young to marry, that the couple are so closely related that the marriage would be incestuous, or that the marriage license is expired -- or other serious emergencies -- such as evidence of infidelity (sexual or otherwise) that might change one of the participants' minds about their continued commitment to their spouse-to-be. In movies and fiction, this is usually a dramatic moment used for the climax of a critical scene. Regardless, it is an incredibly serious objection to raise, and should not be done so lightly. However, the title text describes a confusing and mundane scenario where the only reason the speaker is objecting to the wedding is because they're unsure whether the marriage would make one of the participants their brother-in-law and thus wouldn't know what to call the groom after the wedding.  In order to avoid their own confusion, they attempted to stop the wedding altogether. The officiator rightly ruled that this objection was neither just cause to object nor a reason that the wedding would be unlawful, and is therefore no reason the couple should be prevented from their own chance at wedded bliss.
Since the title text begins with a FYI (for your information) it is implied that Randall has actually tried to stop a wedding using that reason and has been overruled, and thus he wishes to help others avoid that socially-awkward experience.
[A single layer of a family tree with 15 people depicted as sticky figures side by side is shown. They are connected alternated either by a bracket on top or a short line between them. The four outermost figures on each side are faded out in gray.] [In the middle is Cueball and from below an arrow points at him:] Me [To the left Ponytail is connected by a bracket and the arrow below says:] Sibling [To the right of Cueball Megan is shown connected by a small line, an arrow below her reads:] Spouse [Further to the left and the right the next figures have an arrow below with the nested text:] Siblings-in-law [The next connected figures on both sides are drawn with two other wider arrows embedding this statement:] Also siblings-in-law, I think? [All remaining figures left and right have similar arrows below and the text is:] Possible additional siblings-in-law???
[Caption below the frame:] People complain that “<X> th cousin <Y> times removed” is hard to understand, but to me the most confusing one is sibling-in-law, because it chains across both sibling and marriage links and I don't really know where it stops.
