nah ya got it twisted...hes marrying "alayne" to harrold hardyng, whos the next in line to rule the vale of arryn if robbert dies. and after the wedding, hes going to conveniently reveal that alayne is really sansa stark , oldest child living of nerd stark and heir to winterfell
this is from the a wiki of ice and fire to clear things up a bit more
and if you guys recall, littlefinger is already trying to kill robert arryin on the low. remember him telling the maester to give Robbert more medicine (forgot what kind) and the maester was like hes scared because it might kill robbert and littlefinger is all like " dont worry breh, just give him a bit more"
There we go! I knew I wasn't bugging. I remember him explaining to her why she had to marry someone else and I know he wasn't talking about that annoying dying brat.