Stream Aggregate for Sort?
To answer your first question:


  Why is there a Stream Aggregate in the query when I dont have a group by. I am guess it has something to do with the join being merge and its doing a sort?


SQL Server is effectively rewriting your query. This query:

select Object1.Column1
FROM Object2 Object3 WITH (ROWLOCK)
INNER JOIN Object4 Object1 WITH (ROWLOCK) ON Object3.Column2 = Object1.Column1
WHERE Object3.Column2 in (SELECT Column2 FROM Object5)
or Object3.Column3 in (SELECT Column2 FROM Object5);


Can also be written like this:

select Object1.Column1
FROM
(
select Object3.Column2
FROM Object2 Object3 WITH (ROWLOCK)
WHERE Object3.Column2 in (SELECT Column2 FROM Object5)

UNION

select Object3.Column2
FROM Object2 Object3 WITH (ROWLOCK)
WHERE Object3.Column3 in (SELECT Column2 FROM Object5)
) Object3 
INNER JOIN Object4 Object1 WITH (ROWLOCK) ON Object3.Column2 = Object1.Column1;


That's why you have two different access methods on Object2 in the query plan. The Merge Join (Concatenation) operation isn't actually a merge join. It's just implementing a UNION ALL and combining the results. The stream aggregate that you mentioned groups by Object3.Column2. This both removes duplicates from the Merge Join (Concatenation) and sorts the data so that it can be used in the following MERGE JOIN to Object4.

To answer your second question:


  Secondly and more importantly why does an estimate of 13 going
  into the stream aggregate comes out as an estimate of 24,595,900? This is causing the secondary problem of Object4 getting Clustered index scanned instead of a nested loop. I had to split the query into two queries instead of using a OR and the join turns into a nested loop and seeks into Object4.


It looks like a bug. After some searching I found an article by Paul White explaining the issue. The bug is reported to Connect if you want to vote for it or add yourself as affected.

In short, the cardinality estimate is 24595900 because 27328800 (table cardinality of Object2) * 0.9 = 24595900 after rounding. You're on SQL Server 2008 so the legacy CE calculation is used:


  A similar issue occurs with the pre-2014 cardinality estimator, but the final estimate is instead fixed at 90% of the estimated semi join input (for entertaining reasons related to a inversed fixed 10% predicate estimate that is too much of a diversion to get into).


I recommend reading through the entire article. The issue is a little difficult to summarize, but I will attempt to do so by quoting part of the text near the end:


  The cardinality estimator uses the Fixed Join calculator with 100% selectivity. As a consequence, the estimated output cardinality of the semi join is the same as its input, meaning all 113443 rows from the history table are expected to qualify.
  
  The exact nature of the bug is that the semi join selectivity computation misses any predicates positioned beyond a union all in the input tree. In the illustration below, the lack of predicates on the semi join itself is taken to mean every row will qualify; it ignores the effect of predicates below the concatenation (union all).
  
  A similar issue occurs with the pre-2014 cardinality estimator, but the final estimate is instead fixed at 90% of the estimated semi join input (for entertaining reasons related to a inversed fixed 10% predicate estimate that is too much of a diversion to get into).


Splitting up the query as you did should prevent the issue. You also might have some luck by rewriting the query using UNION (see Example 4 in the article).