Complex sentence: Elizabeth's cousin, Mary Queen of Scots, aged 19 and a devout Catholic, had returned from France to assume her disastrous reign in Scotland, and by 1569 been forced to abdicate and was on the run.
Split into simpler sentences: Elizabeth's cousin, Mary Queen of Scots, aged 19 and a devout Catholic, had returned from France to assume her disastrous reign in Scotland. The staunchly Catholic Queen Mary had been forced to abdicate and was on the run.